So a lot of people have been asking for this so I made sure I got in a few games both with and against Dark Eldar this week. Remember, this is an initial analysis.
Dark Eldar were the ultimate glass cannon in 5th edition. They had good firepower but died to a stiff breeze in return with generally weak infantry (T3/5+) and vehicles (AV10, open-topped). None of this has really changed but their vehicles didn't really get any weaker. Against individual shooting, AV10, open-topped is just as survivable as a closed vehicle was in 5th edition. Hull points obviously makes them more vulnerable to being destroyed over time but well every army has them and Dark Eldar were more likely to be destroyed from two penetrating hits before anyway. So in essence, most of their vehicles went up in durability RELATIVELY to the rest of the game whilst Venoms stayed the same (only two hull points). Their flyers went up a lot obviously needing to be hit on 6′s but so did other flyers.
At the same time, due to general AV10 across the board, small-arms fire is actually more dangerous due to hull points. Previously, it took, on average, six glancing hits to remove an AV10, open-topped vehicle (needing that straight 6) but now with hull points Venoms need take only two glancing hits and Raiders three from your bog standard bolters. That's a squad of Marines against a Venom so protecting yourself against small arms fire has increased in importance *looks at Night Shields*.
Furthermore, every vehicle now has jink which means flickerfields are only really needed against combat situations. This saves some points (or you can switch them to nightfields) and ensures every vehicle will get a cover save as long as it has moved the previous turn. Even in the open. Although cover saves are less than they were before, they are a lot easier to get. The reduction hurts Dark Eldar a bit more as every shot which goes through is more likely to hurt but it's nice to know every army is in the same boat in this regard.
Power from the Pain gets a bit of a hit as well. FNP is less useful on the average Dark Eldar unit as S6, which still ignores FNP, is quite common and AP1/2 weapons aren't as huge a threat to Dark Eldar as they are say Space Marines. The overall reduction in FNP to 5+ is generally not offset by the ability to take FNP against AP1/2 weapons in this case due to the lower average toughness of the Dark Eldar units. Furious Charge is also not as good as it once was though Dark Eldar generally strike before opponents anyway. Fearless however is quite a bit better with the removal of No Retreat – getting Fearless onto Beast units now is quite important but for the most part, Dark Eldar don't have large spammy units which could really benefit from the Fearless USR as a tarpit.
Assaulting from transports isn't as useful either with the essential 3″ reduction. You can still move 6″, jump out 6″ and assault 2D6″ but you've lost the 2.99″ gained from moving and disembarking. Dark Eldar vehicles are at least open-topped so aren't slapped around like a lot of armies but they are generally a more offensive than counter-assault army anyway… Fleet on a lot of their units helps take full advantage of that 2D6″ though. Fleet through cover is more reliable than straight up 2D6″ – take note any assault units who wish to be scary. Without Fleet, you're not and luckily for most Dark Eldar assault units, they do have Fleet. Unluckily, most of them now suck due to changes in power weapons with only Beastmasters staying atop the pile. Wyches are still decent anti-infantry as they always were but Agonisers now are very expensive and not so great which really forces them still into the torrent department.
Haywire however has been improved drastically in the damage it can do thanks to hull points. So Wyches with grenades (which can also be thrown) and anything with Haywire blasters (Scourges, looking at you) got a really nice boost. Grenades in particularly since tanks are, at worst, hit on a 3+.
Darklight weapons are finally 'better' than missile launchers at dropping vehicles thanks to AP2. They don't have the anti-infantry duality missile launchers bring but they retain their 1/3 chance to destroy a vehicle outright whilst everyone else has been downgraded to 1/6. This is pretty nice, particularly considering most Dark Eldar armies were running 20+ Darklight weapons which was decent anti-tank before but ran up against the serious issue of stun-locking everything and getting attritioned away. Now thanks to Hull points this doesn't happen and if they roll average they're going to kill those extra tanks due to their AP2.
Allies is where Dark Eldar really struggle. A lot of other armies who regressed from 5th or had issues with 5th or were just good in 5th got a lot more options with allies. Dark Eldar, like most of the super evil stuff, didn't. Their only real beneficiary here is Eldar and well Eldar suffer from the same major issues Dark Eldar do – weak Troops. That being said, being able to take six small Troops (i.e. Warriors or Wracks) and then two larger Troops from Eldar does have it's advantages, it's just not the same as eight Troops of Marines who can also combat squad, etc.
To be honest, I'm not sure. The initial reaction is OMG DARK ELDAR SUCK now. I wouldn't say they got better with 6th but I wouldn't say they got drastically worse. In relation to other vehicles they didn't lose as much but only two hull points on Venoms and being vulnerable to basic S4 weapons is pretty shitty all-round. Throw in open-topped and even late game your vehicles aren't as survivable as others late-game. Yes they had this before but… Chuck in the relatively poor Troops and limited access to allies and I'd say they stayed even whilst other armies got a little bit better generally due to allies. The general concept of lots of Venoms backed up by Ravagers & token Raiders with Fast Attack filler (i.e. Beastmasters + Scourges) is basically the same in terms of what Dark Eldar can run with the potential addition of extra Eldar scoring, Fire Dragons, D-Cannons & a Farseer. The Quin-star is cute but it's still a deathstar which has counters, etc.
There's still more to uncover here which hopefully more games will unlock but I generally feel Dark Eldar are running in place in 6th edition. Their basic build still works and they are still a glass cannon but their most limiting aspect of 6th is their real lack of super good ally choices. If Eldar get a good update this may change… We shall see. What have others experienced?