Angels know God better than Christians but through the latter angels know God's manifold wisdom

This comes out in Ephesians 3:10, 11. See also 1 Peter 1:12.
Ephesians 3:10, 11
His intent was that now, through the church, the manifold wisdom of God should be made known to the rulers and authorities in the heavenly realms, 11 according to his eternal purpose that he accomplished in Christ Jesus our Lord.

Angels are better than humans but converted humans are better than angels

This comes out firstly in Psalm 8:5 and Hebrews 2:7 and then Hebrewss 1:14, 1 Corinthians 6:3, etc. Angels are created superiror to humans but in the new heavens and earth believing humans will reign over angels.
Psalm 8:5 You have made them a little lower than the angels and crowned them with glory and honour.
Hebrews 1:14 Are not all angels ministering spirits sent to serve those who will inherit salvation?

Angels of Light can be masquerading messengers of Satan

Talking about the deceitfulness of false teachers Paul says

2 Corinthians 11:14 And no wonder, for Satan himself masquerades as an angel of light.

Angels often mentioned in Scripture

According to C Fred Dickason (Angels Elect and Evil) there are references to angels in 34 of the 66 books of the Bible (17 in the Old Testament, 17 in the New Testament). The word appears 108 times in the Old Testament and 165 times in the New Testament.

Angels are ministering spirits sent to serve those who will inherit salvation

Hebrews 1:14 Are not all angels ministering spirits sent to serve those who will inherit salvation?
On this verse Bishop Ellicott comments

In this verse and the preceding is repeated the contrast of Hebrews 1:7-9, in reversed order. The words "ministering spirits" at once recall the "ministers" and "winds" (expressed in Greek and Hebrew by the same word as "spirits") spoken of in Hebrews 1:7. In the LXX. this word "minister" is usually applied to those who stood before God in His earthly sanctuary: so here it is fitly used of the nobler offices of the unseen world. To the English reader it may seem that those who in Hebrews 1:7 are God's ministers are here represented as servants of man. It is not really so, for the words properly mean ... sent forth (that is, continually sent forth) to do service (to God), for the sake of them who are to inherit salvation. "Inherit" is a prelude of Hebrews 2:10. The last word, "salvation," expresses the divine purpose indicated by all the prophecies that have passed under review. The chapter has been occupied with promises of the Christ: the last word brings before us Jesus, the Saviour.

Barnes says

In this doctrine there is nothing absurd. It is no more improbable that angels should be employed to aid man, than that one man should aid another; certainly not as improbable as that the Son of God should come down "not to be ministered unto but to minister," Matthew 20:28, and that he performed on earth the office of a servant; John 13:1-15. Indeed it is a great principle of the divine administration that one class of God's creatures are to minister to others; that one is to aid another to assist him in trouble, to provide for him when poor, and to counsel him in perplexity. We are constantly deriving benefit from others, and are dependent on their counsel and help. Thus, God has appointed parents to aid their children; neighbours to aid their neighbours: the rich to aid the poor; and all over the world the principle is seen, that one is to derive benefit from the aid of others. Why may not the angels be employed in this service?
They are pure, benevolent, powerful; and as man was ruined in the fall by the temptation offered by one of an angelic, though fallen nature, why should not others of angelic, unfallen holiness come to assist in repairing the evils which their fallen, guilty brethren have inflicted on the race? To me there seems to be a beautiful propriety in bringing "aid" from another race, as "ruin" came from another race; and that as those endowed with angelic might, though with fiendish malignity, ruined man, those with angelic might, but heavenly benevolence, should aid in his recovery and salvation. Further, it is, from the necessity of the case, a great principle, that the weak shall be aided by the strong; the ignorant by the enlightened; the impure by the pure; the tempted by those who have not fallen by temptation. All over the world we see this in operation; and it constitutes the beauty of the moral arrangements on the earth; and why shall not this be extended to the inhabitants of other abodes? Why shall not angels, with their superior intelligence, benevolence, and power, come in to perfect this system, and show how much adapted it is to glorify God? In regard to the ways in which angels become ministering spirits to the heirs of salvation, the Scriptures have not fully informed us, but facts are mentioned which will furnish some light on this inquiry. What they do now may be learned from the Scripture account of what they have done - as it seems to be a fair principle of interpretation that they are engaged in substantially the same employment in which they have ever been. The following methods of angelic interposition in behalf of man are noted in the Scriptures:
(1) They feel a deep interest in man. Thus, the Saviour says, "there is joy in the presence of the angels of God over one sinner that repenteth;" Luke 15:10. Thus also he says, when speaking of the "little ones" that compose his church, "in heaven their angels do always behold the face of my Father who is in heaven;" Matthew 18:10.
(2) they feel a special interest in all that relates to the redemption of man. Thus, Peter says of the things pertaining to redemption, "which things the angels desire to look into;" 1 Peter 1:12. In accordance with this they are represented as praising God over the fields of Bethlehem, where the shepherds were to whom it was announced that a Saviour was born Luke 2:13; an angel announced to Mary that she would be the mother of the Messiah Luke 1:26; an angel declared to the shepherds that He was born Luke 2:10; the angels came and ministered to Him in His temptation Matthew 4:11; an angel strengthened Him in the garden of Gethsemane Luke 22:43; angels were present in the sepulchre where the Lord Jesus had been laid, to announce His resurrection to His disciples John 20:12; and they reappeared to his disciples on Mount Olivet to assure them that he would return and receive his people to him self, Acts 1:10.
(3) they appear for the defence and protection of the people of God. Thus it is said Psalm 34:7, "The angel of the Lord encampeth round about them that fear him, and delivereth them." Thus, two angels came to hasten Lot from the cities of the plain, and to rescue him from the impending destruction; Genesis 19:1,15. Thus, an angel opened the prison doors of the apostles, and delivered them when they had been confined by the Jews; Acts 5:19. Thus, the angel of the Lord delivered Peter from prison when he had been confined by Herod; Acts 12:7-8.
(4) angels are sent to give us strength to resist temptation. Aid was thus furnished to the Redeemer in the garden of Gethsemane, when there "appeared an angel from heaven strengthening him;" Luke 22:43. The great trial there seems to have been somehow connected with temptation; some influence of the power of darkness, or of the Prince of evil; Luke 22:53; compare John 14:30. In this aid which they rendered to the tempted Redeemer, and in the assistance which they render to us when tempted, there is a special fitness and propriety. Man was at first tempted by a fallen angel. No small part - if not all the temptations in the world - are under the direction now of fallen angels. They roam at large "seeking whom they may devour;" 1 Peter 5:8. The temptations which occur in life, the numerous allurements which beset our path, all have the marks of being under the control of dark and malignant spirits. What, therefore, can be more appropriate than for the pure angels of God to interpose and aid man against the skill and wiles of their fallen and malignant fellow-spirits? Fallen angelic power and skill - power and skill far above the capability and the strength of man - are employed to ruin us, and how desirable is it for like power and skill, under the guidance of benevolence, to come in to aid us!
(5) they support us in affliction. Thus, an angel brought a cheering message to Daniel; the angels were present to give comfort to the disciples of the Saviour when he had been taken from them by death, and when he ascended to heaven. Why may it not be so now, that important consolations, in some way, are imparted to us by angelic influence? And,
(6) they attend dying saints, and conduct them to glory. Thus, the Saviour says of Lazarus that when he died he was "carried by the angels into Abraham's bosom;" Luke 16:22. Is there any impropriety in supposing that the same thing may be done still? Assuredly, if anywhere heavenly aid is needed, it is when the spirit leaves the body. If anywhere a guide is needed, it is when the ransomed soul goes up the unknown path to God. And if angels are employed on any messages of mercy to mankind, it is proper that it should be when life is closing, and the spirit is about to ascend to heaven. Should it be said that they are invisible, and that it is difficult to conceive how we can be aided by beings whom we never see, I answer, I know that they are unseen. They no longer appear as they once did to be the visible protectors and defenders of the people of God. But no small part of the aid which we receive from others comes from sources unseen by us. We owe more to unseen benefactors than to those whom we see, and the most grateful of all aid, perhaps, is what is furnished by a hand which we do not see, and from quarters which we cannot trace. How many an orphan is benefited by some unseen and unknown benefactor! So it may be a part of the great arrangements of Divine Providence that many of the most needed and acceptable interpositions for our welfare should come to us from invisible sources, and be conveyed to us from God by unseen hands.

Angels are condemned with Satan if they are evil

Matthew 25:41 Then he will say to those on his left, 'Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels.' 

The cursed, fallen, evil angels will one day be condemned with their leader Satan and be cast into the eternal fire prepared for them.

Angels holy and unholy

There are at least three references to holy angels in the New Testament.
Mark 8:38  If anyone is ashamed of me and my words in this adulterous and sinful generation, the Son of Man will be ashamed of them when he comes in his Father's glory with the holy angels.
Luke 9:26  Whoever is ashamed of me and my words, the Son of Man will be ashamed of them when he comes in his glory and in the glory of the Father and of the holy angels. 
Revelation 14:10  ... they, too, will drink the wine of God's fury, which has been poured full strength into the cup of his wrath. They will be tormented with burning sulfur in the presence of the holy angels and of the Lamb.
By implication fallen, non-elect angels are not holy. The holy angels, Gill notes, will be judges and will condemn unbelievers to everlasting burnings and pronounce the sentence on them. They will gather them out and seperate them from the righteous, cast them into the furnace of fire, will be spectators of their punishment and rejoice at it. The sight of their power and glory will increase the torment of the sufferers.

Angels reveal prophetic truth from God

 This comes out i Daniel 10 as well as in other places such as Ezekiel, Zechariah and Revelation.

Angels can be encountered without our realising

Hebrews 13:2 Do not forget to show hospitality to strangers, for by so doing some people have shown hospitality to angels without knowing it.
This is clearly a reference back to Abraham and Lot back in Genesis 18 and 19 and is intended to encourage hospitality. However, it does suggest that it is possible to meet an angel and not know, for them to appear incognito. Perhaps the idea is of the stranger brining the angels with him.

Angels rejoice when sinners repent

In Luke 15:10 it says that

... there is rejoicing in the presence of the angels of God over one sinner who repents.

If that is read carefully, it must mean thhat God rejooices when sinners repent. Undoubtedly the angels rejoice too.

Angels can be elect or non-elect

1 Timothy 5:21 I charge you, in the sight of God and Christ Jesus and the elect angels, to keep these instructions without partiality, and to do nothing out of favouritism.
Paul refers to elect angels, implying that there are also non-elect angels. Calvin says (Institutes 3.23.4) "The angels who have persevered in their upright state are called "elect" by Paul (1 Tim 5:21); if their steadfastness was grounded in the good pleasure of God, then the apostasy of the other angels shows these were forsaken. For that no other cause can be adduced than reprobation, which is hidden in God's secret counsel." This election must have been supralapsarian (logically before the fall) according to Geerhardus Vos. He suggests this election must be in Christ and in Christ as ruler and head (not Mediator). The elect angels did not fall but the non-elect did, according to the will and purpose of God.

Angels good and bad; some lost their positions of authority, some did not

Jude 1:6 And the angels who did not keep their positions of authority but abandoned their proper dwelling - these he has kept in darkness, bound with everlasting chains for judgment on the great Day.

2 Peter 2:4 For if God did not spare angels when they sinned, but sent them to hell, putting them in chains of darkness to be held for judgment; 

Some angels are good and some are bad, the demons or devils. They are described here as those who sinned and so did not keep their positions of authority but abandoned their proper dwelling (heaven?) and were sent to hell. God has kept them in darkness, bound with everlasting chains for judgment on the great Day. They may seem unchained but they are not really. When were they actually hurled down to hell? Probably when Christ died on the cross.

Angels are not to be believed if they say what is false

Galatians 1:8 But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let them be under God's curse!
Paul is no doubt speaking of a hypothetical situation, although there is every reason to suppose a fallen angel preaching a gospel other than the one ... preached to the Galatians by Paul. As glorious and wonderful as angels are they are only to be listened to as far as they speak God's word.

Angels have a special commission to guard believers

Psalm 91:11, 12  For he will command his angels concerning you to guard you in all your ways; they will lift you up in their hands, so that you will not strike your foot against a stone.

These are the verses that Satan tried to misuse when he tempted Jesus in the wilderness. We are not to tempt God but we are to be confident that his angels will guard us in all our ways, if we are believers. They will make sure we come to no harm.

Spurgeon says that rather than having "one guardian angel, as some fondly dream" all the angels are active on our behalf. He calls them "the bodyguard of the princes of the blood imperial of heaven".
"It is down in the marching orders of the hosts of heaven that they take special note of the people who dwell in God. It is not to be wondered at that the servants are bidden to be careful of the comfort of their Master's guests; and we may be quite sure that when they are specially charged by the Lord himself they will carefully discharge the duty imposed upon them."
"The protection here promised is exceeding broad as to place, for it refers to all our ways, and what do we wish for more? How angels thus keep us we cannot tell. Whether they repel demons, counteract spiritual plots, or even ward off the subtler physical forces of disease, we do not know. Perhaps we shall one day stand amazed at the multiplied services which the unseen bands have rendered to us."
"as nurses carry little children, with careful love, so shall those glorious spirits upbear each individual believer ... even minor ills they ward off. It is most desirable that we should not stumble, but as the way is rough, it is most gracious on the Lord's part to send his servants to bear us up above the loose pebbles. If we cannot have the way smoothed it answers every purpose if we have angels to bear us up in their hands. Since the greatest ills may arise out of little accidents, it shews the wisdom of the Lord that from the smaller evils we are protected."

Angels are humbly slow to assert their own authority

Jude 1:9 But even the archangel Michael, when he was disputing with the devil about the body of Moses, did not himself dare to condemn him for slander but said, "The Lord rebuke you!" 

2 Peter 2:11  yet even angels, although they are stronger and more powerful, do not heap abuse on such beings when bringing judgment on them from the Lord. 

The allusion is an obscure one but the point is clear. Even Michael the Archangel, the head of all angels, did not take it upon himself to rebuke Satan or condemn his slander but meekly said "The Lord rebuke you!" . All the angels show the same modesty. In many cases they are stronger and more powerful then the fallen angels who oppose them but they do not heap abuse on such beings even when bringing judgment on them from the Lord. 

Angels praise God

Psalm 103:20-21 Praise the LORD, you his angels, you mighty ones who do his bidding, who obey his word. Praise the LORD, all his heavenly hosts, you his servants who do his will.
(See also Psalm 148:2 Praise him, all his angels; praise him, all his heavenly hosts.)

Angels have good reason to praise God - For their creation by God, for their preservation, for their happiness, in which they continue as they are elect of God in Christ.
They are always employed in the work of praising the Lord nor are they in the least backward in it, nor remiss. That is not why they are addressed as they are. Rather the design of the psalmist is only to show how great and good the Lord is and that angels, as great as they are, are under an obligation to praise him, and are bound to do so. He also intends to stir himself and others to this work, from such a consideration - that such noble creatures praise God.
C H Spurgeon
Finding his work of praise growing upon his hands, he calls upon "the firstborn sons of light" to speak the praises of the Lord, as well they may, for as Milton says, they best can tell. Dwelling nearer to that prepared throne than we as yet have leave to climb, they see in nearer vision the glory which we would adore. To them is given an exceeding might of intellect, and voice, and force which they delight to use in sacred services for him; let them now turn all their strength into that solemn song which we would send up to the third heaven. To him who gave angelic strength let all angelic strength be given. They are his angels, and therefore they are not loath to ring out his praises.

Angels will be judged one day - by believers

Paul says

1 Cor 6:3 Do you not know that we will judge angels? How much more the things of this life!

There is some mystery here but at the very least we can say that some angels will be judged and that believers will have some part in that judgement. No doubt that part will be a confirmatory one rather than anything more reliant on our own decisions. The fallen angels will be condemned to hell and the good angels will be confirmed for heaven. No doubt the focus lies on the fallen angels.

Angels revealed and announced the resurrection of Christ

Matthew 28:2-7 There was a violent earthquake, for an angel of the Lord came down from heaven and, going to the tomb, rolled back the stone and sat on it. His appearance was like lightning, and his clothes were white as snow. The guards were so afraid of him that they shook and became like dead men. The angel said to the women, "Do not be afraid, for I know that you are looking for Jesus, who was crucified. He is not here; he has risen, just as he said. Come and see the place where he lay. Then go quickly and tell his disciples: 'He has risen from the dead and is going ahead of you into Galilee. There you will see him.' Now I have told you."

Luke 24:2-7 They found the stone rolled away from the tomb, but when they entered, they did not find the body of the Lord Jesus. While they were wondering about this, suddenly two men in clothes that gleamed like lightning stood beside them. In their fright the women bowed down with their faces to the ground, but the men said to them, "Why do you look for the living among the dead? He is not here; he has risen! Remember how he told you, while he was still with you in Galilee: 'The Son of Man must be delivered over to the hands of sinners, be crucified and on the third day be raised again.'"

Angels - their negative work

1. Striking the Sodomites with blindness Genesis 19:11 Then they struck the men who were at the door of the house, young and old, with blindness so that they could not find the door.
2. Striking dead rebels in the wilderness in Moses' time 1 Corinthians 10:10 And do not grumble, as some of them did - and were killed by the destroying angel.
3. Willingness to slay Balaam by sword Numbers 22:31 Then the LORD opened Balaam's eyes, and he saw the angel of the LORD standing in the road with his sword drawn. So he bowed low and fell facedown. 
4. Driving out the Canaanites Exodus 33:2 I will send an angel before you and drive out the Canaanites, Amorites, Hittites, Perizzites, Hivites and Jebusites. 
5. Beginning to destroy the Israelites in David's time 1 Chronicles 21:15 And God sent an angel to destroy Jerusalem. But as the angel was doing so, the LORD saw it and relented concerning the disaster and said to the angel who was destroying the people, "Enough! Withdraw your hand." The angel of the LORD was then standing at the threshing floor of Araunah the Jebusite. (See 2 Samuel 24:16)
6. Slaughtering the Assyrians 2 Kings 19:35 That night the angel of the LORD went out and put to death a hundred and eighty-five thousand in the Assyrian camp. When the people got up the next morning - there were all the dead bodies! (See 2 Chronicles 32:21, Isaiah 37:36)
7. Striking Zechariah mute Luke 1:19, 20 The angel said to him, "I am Gabriel. I stand in the presence of God, and I have been sent to speak to you and to tell you this good news. And now you will be silent and not able to speak until the day this happens, because you did not believe my words, which will come true at their appointed time." 
8. Striking Herod dead Acts 12:23 Immediately, because Herod did not give praise to God, an angel of the Lord struck him down, and he was eaten by worms and died.
9. More generally Psalm 35:5, 6 May they be like chaff before the wind, with the angel of the LORD driving them away; may their path be dark and slippery, with the angel of the LORD pursuing them.
10. Various judgements Revelation 8;5 etc Then the angel took the censer, filled it with fire from the altar, and hurled it on the earth; and there came peals of thunder, rumblings, flashes of lightning and an earthquake. (See also Revelation 16, etc).

Angels are referred to together as a heavenly host

Psalm 103:20, 21 Praise the LORD, you his angels, you mighty ones who do his bidding, who obey his word. Praise the LORD, all his heavenly hosts, you his servants who do his will.

Psalm 148:2  Praise him, all his angels; praise him, all his heavenly hosts.

The idea of hosts is also attached to armies and to stars. The angels form a great army, an army of light. (Cf Job 38:7; Revelation 9:1, 11).