I don't know all the Chaos crap thats out there; but I can tell you about a few units/models that have some serious point values and crumble well to Dire Avengers that are worth a lot less.
5 Termies (250ish), vs 10 DA, exarch with Bladestorm, pw & ss (157)
27 shots * 2/3 hits * 1/2 wounds * 1/6 kills = 1.5 Dead,
assault for 21 attacks * 1/2 Hits * 1/3 wounds * 1/6 kills = 0.58 more dead,,,
for a total of 2.25 removed Termies (without DOOM), adding in DOOM will make the result closer to 4 removed.
The key with DA (as with all other Eldar units) is to use their strengths and avoid their weeknesses. Don't send DA in to fight against enemy CC specialists, and don't leave them to get shot in the open.... Don't shoot things that can be shot by lesser units with the same effect, don't Bladestorm if it leaves you stranded, etc, etc,.... you get the picture..... Figure out what the DA are good at and then use them for that, regardless of enemy. Oh, and I think that their bet attribute is that they can shoot pretty near anything and do well, they are not as good at HtH, so this is where they need to be supported. Here is an Example.... Some-one noted that they can drop about 3-4 Necrons with their shooting (so add DOOM to enhance this, then have a squad of Harlequins (or other CC unit) run over that same unit of Necrons; if you drop the whole unit, there will be less of them left to threaten your DA later. Oh and hte same could be done with multiple DA squads: two squads of DA both in Serpents can easily drop in and waste 10 Necron Warriors (with the Serpents shooting some weapons to assist), that means that there maybe 5 necrons after WBB rolls (if they are not outside of the WBB roll distance from another simular unit). You need to consider the entire amries synergy wehn you discuss Eldar tactics, as the Eldar do not work nearly as well as individual units as they do within combined efforts.
EDITED 13 Sep 2007: I adjusted the math for 27 shots with Bladestorm,,,, wow what a difference (0.17 less kills overall), WoW there will still be about 2.25 Termies removed (or closer to 4 dead if you happen to have DOOM'ed them as well). Thanks for the correction though; even if its only an average since the dice are never exactly the same as the Real World Math.