One of the few remaining armies we haven't done and sparked on a bit by some BoLS comments. Apparently Eldar are the best thing ever...*eyebrow*. Right, let's get to it.
S6, speed and psychic might were and still are the Eldar's main strengths. One of their best strengths - game long durability in vehicles though, got slapped. Luckily Allies are there and Eldar didn't get shafted in that regard. Hull Points means Holo-Falcons are all but a waste of points and the super duper Wave Serpents running around and laughing at melta-weapons with a 5th ed vehicle damage table are done. They're still durable like all vehicles but Hull Points means they are much more likely to die. Holo-fields are also ass now - aw look, you're still dying despite paying a bunch of points not to - hull points killed this upgrade. Without being able to reserve your entire army, you cannot play a game of "you cannot" hope to kill me before the game ends. Wasn't a good idea before but now it's not really at all viable.
Don't worry though, Vypers, like Venoms got a tiny bit more durable overall though are more susectipble to light arms firepower (S4 guns) and don't have the nightfield upgrade to limit this. The standard Eldar vehicle at AV12/12/10 and three HP is still pretty durable and with the added bonus of Jink, you'll always get some sort of cover save on those often hard to hide Eldar vehicles. Unfortunately though, vehicles now no longer contest objectives and Troops have to be outside transports to hold objectives. This means the great expense of lots of Wave Serpents takes away from actually being able to score AND deny your opponent's scoring in the very common objective games. With so many points sunk into this, well the Eldar can be on the back foot immediately and their foot units haven't somehow gained in durability or firepower with the addition change.
This can be a real pain for Eldar as well in that more points need to be invested in Troops - and generally larger Troops squads to ensure they can actually hop out on objectives and weather some firepower (the beauties of Random Game Length). Wraithguard squads as Troops can obviously be more useful here but aren't cheap and still roll over to halfway decent combat units in assault. Large Guardian squads are large Guardian squads - slow-moving, minimum ranged firepower for cost and terrible in combat. At least Avatar fearless bubble isn't a liability all the time now. Rangers/Pathfinders are still small squads with reduced cover though their damage capacity has gone up somewhat but they are just too expensive to get large squads for. Allies open up several options here at least - Eldar can access everyone but Chaos Marines/Daemons, Necrons & Tyranids though Sisters & Orks are Desperate. Pretty good array of durable scoring Troops there with all the Marines armies + Imperial Guard (bodies & firepower in scoring rather than T4/3+).
S6 firepower really got an improvement - all those Glances are actually killing vehicles now though your suppression isn't as reliable. Still, that many S6 shots will bring down even AV12 at some point but is quite effective against the lower AVs. The issue of course is AV13+, Eldar still really only have one answer and that's Fire Dragons. EMLs & Bright Lances exist yes but they are expensive and single shots - not exactly setting the world on fire, particularly given the range of S8+ weapons other armies have. If only the Pulse Laser was a more common sight as a weapon...
Eldar Aspects are still pretty ass for the most part and now all the assault units are really bad since they cannot assault out of an even stationary vehicle. Dark Reapers get a bit of a boost with the cover reduction but are still very expensive and immobile though they can strip hull points rather than suppressing tanks at least now. Warp Spiders are likewise decent at this but they do exactly what the rest of the army does but at a much shorter range - S6 spam. Fire Dragons are still good of course though the inability to move 12" and disembark hurts them, Flat Out moves from Serpents can mitigate this a lot and can make them much more than a one hit wonder - if you want to sacrifice your Serpent's shooting for this. An expensive proposition but if it keeps one of the best anti-tank units in the game alive a turn longer, generally worthwhile.
Which of course takes us to mobility - Eldar are still one of the darlings there to be sure. With fast skimmers they can move and fire at a good clip and have good turbo-boosting options. The only downside of course is if you want to get passengers out, your movement becomes quite restricted if you want to maintain your firepower (i.e. not turbo-boost to maintain movement). With a larger foot basis ultimately required at some point in the game to score objectives, this can hinder one of Eldar's greatest strengths and force a less mech-centric approach to army building unless one is looking to simply drop weak Troops on objectives late games (which as we discussed above, has issues).
Harlequins are of course the darlings of 6th edition with Allies and bringing in a disgusting amount of USRs + Fortune to make the famed Harlie-star. Shame the usual LoS blocking tactics to Characters to limit LoS! rolls, cover ignoring weapons, fearless tarpits, psy defense, etc. can stop this. Yes, it's hard to deal with face-up, 2+ re-rollable cover with lots of Rending attacks in combat is a pain but if you want a massive star with lots of characters with the proper USRs/abilities (i.e. shadowfields), you're looking at 1000 points of your army. In the end it's the same as before but much less vulnerable to being double-tapped off the board and with the added benefit of USR transference. This same USR transference can help the old Eldar deathstar - the Seer Council. Getting Hit & Run through Dark Eldar or Tau Allies makes this unit a lot more viable but still the same old in terms of relying on Fortune, etc.
Eldar still has some great utility in Runes of Warding and for anyone not relying on psychic powers, this makes them a great Ally as it not only stops psychic powers, it actively encourages individuals not to flex their psychic abilities due to increased chances of Perils. With access to the much lauded Divination psychic discipline, this makes Eldar another great Ally choice in terms of HQs - they only lack really in the Troop department.
Conclusion
Eldar haven't really gained in 6th edition and although their utility as Allies in HQ is pretty nice, their Troops aren't great. Yes you can get a backfield scoring Ranger/Pathfinder unit but you don't want Allies for that... Otherwise their standard Mechdar build isn't very viable at all anymore and a more Hybrid style, or at least units which can get out and play, is going to be more likely with Allied in units to give them a more durable base. You're building on a weaker structure though unfortunately so Eldar are going to be working more often as Allies I imagine.
