I have seen lately a plethora of blog articles on building competitive dark eldar list - typically they all have a title something along the line of being all you need to know about dark eldar. I read them and find that most delve into fielding min/maxed spammed lists that follow along the line of list building originally proposed by Thor665 over on Dakka Dakka... That is lots of five man squads of Warriors with a blaster riding in raiders, two assault units (take your pick - typically Incubi) and three Ravagers. It's the same old thing repackaged as something new. Here is a link to one of Thor's tactica articles on dark eldar, it's really good stuff:
One thing I want to point out is that I have yet to see any of the blogosphere gurus claiming to have won a tournament with dark eldar. When I see this then I'll be the first to tip my hat. Honestly and by no means meaning to come across as rude I think we would all be better served if the gurus would wait until they win a tournament. Then there would certainly be a ton of useful information to share. Another thing I'd like to point out is that there seems to be a prevalent sentiment that mech IG is the bane of dark eldar. People just seem to throw in the towel and say they give up versus IG. Sure it's possible you could attend a tourney with your new dark eldar list and not draw a match versus IG. Is that a sound strategy? No it's not. You've got to bring an army that can beat any other army. I have plenty of experience beating IG. I'd go so far to say I always look forward to playing against IG. I always feel confident I can beat IG. Nope, I'm not by any means suggesting it's an auto win but I do win a lot of the time.
I have been spending a lot of time play testing the new codex. It's nice to play something other than power armor. Unfortunately it seems to me GW has given all the loving to Imperial armies with Daemons being the one true exception. Now we have the dark eldar codex with a new line of fantastic looking minis. The minis are what really pulled me in. I am by nature a competitive player and like to run unique armies. A lot of people who have spent more time than me play testing have said dark eldar are buggered by mech IG. I did a bit of research asking some gamers who profess to smacking mech IG around what characteristics of their armies worked well against the tread heads. Typically I have found that Salamanders, Space Wolves and Blood Angels work the best versus mech IG. These three armies are all Space Marines and have access to lots of melta weapons. If you've played against Salamanders then you know just how brutal are twin linked meltas. These armies can all mech up as well plus Space Marines in general are naturally resilient. Of course you can't play dark eldar like Space Marines BUT you can learn from what works for them. To me and based upon my own play testing I see a diversity in taking different units that work well together is the best approach to building a competitive list (for me). A lot of the net lists I see now are based upon the gunboat list that was prevalent with the old codex / it was lots of five man raider squads with a dark lance, a couple of assault units in raiders and three ravagers. There are lot more good options to choose from now, in fact I think you can do well with only two to three troop choices. The generic Archon and Haemonculus both really buff dark eldar armies. The elite wyches only cost a tad more to field point wise compared to the troop choice and are a lot more potent. Blood Brides are an excellent tarpit versus units such as assault terminators and are effective versus vehicles if you equip them with haywire grenades. Any unit that fulfill more than one role is worth taking in my opinion. I like the reaver jet bikes because they are the most versatile unit. Voidravens you've got a S9 dark lance - that extra point of strength can make a big difference; I'd rather have two S9 shots over three S8 shots.
Phil Kelly wrote the new codex so there must be a viable army. Versus armies like Tyranids and Space Wolves you've got all the tools you need just for two typical examples. Mech IG is definitely a tough nut to crack but I think it can be done and you don't have to rely upon going first. Relying upon going first is another equation for disaster.