I don't know who's Ra this is, but it's AWESOME.
Seers: I know the Seers are the most popular HQs with most Eldar players (though not usually with me). With the new Runes of battle dual-powers the Eldar come into actually being the most powerful psychics in the galaxy. And the Farseer gets mastery 3 for only 100pts, which is awesome. I also like the new Farseer powers -- or should I say, the old ones. Some of them (like death mission) harken back to the very first farseers of the early 90s, which is cool. Now, I have heard some players griping that, "Oh, Eldar took a big blow cuz we can't pick fortune anymore, we have to roll it." My resoonse is, "Tough $#!^." So you don't get your guarenteed cheat anymore. I personally like the random powers, it adds a fun and uncontrolled element to the game. The farseers Runes of Witnessing and Warding have become much less potent and are now one-use expended, which is good for balance but I did like them better before.
We now have 3 seer HQ options, the Farseer, Spiritseer, and the Warlocks. The Farseer I don't need to say much about. The Spiritseer is pretty much wraith-oriented, which will make a lot of folks happy, as he makes wraithguard/swords troops and gives them a markerlight-type ability. He is also a mastery 2 warlock using the same Runes of Battle discipline and a 2 wound model, which is nice (though I don't expect to use one much personally).
The Warlocks have become just about what I hoped they would. They are a non-slot HQ choice you can always take and they work similar to SW Wolf Guard -- you can make a council unit of 1-10 models, branch off one at a time to lead guardian units (troops, bikers, and heavy wpn teams), and keep the rest to form their own unit. I was hoping they'd go so far as Indepenedent Characters like the Blood Angel medic so they could join about anyone, but no such luck. I personally will be having one join my Windrider Bikes and have a Warlocke Triad unit who stick together and help others in the battle. (I've dnoe that once now and I held them back too much to be of any real use). So I'm kind of excited about the Warlock changes.
And of course Eldrad. He finally lives up to the ultimate psyker title too. Mastery 4 and he can regenerate warp charge points while he's casting on a 5+ roll. He's going to be even more popular than before.
Illic Nightspear: This guy is a cool, badass type ninja sniper. Very cool character but... I don't think I'll be giving up a precious HQ spot for him. (I might once just to try him out.) I'm a bit disappointed that taking him is the only way to upgrade rangers to pathfinders, but if you do so they can deepstrike right up next to Nightspear, who can infiltrate without limitations, like they're all crawling out of the bush at just the right spot. I just wish he were a ranger upgrade character rather than an HQ. If you just want to run allies, though, him with some pathfinders would make a cool add-on unit of Outcast snipers.
Autarchs: Pretty much unchanged from the last edition. What I like about them is the Autarch is basically a "make your own hero kit," which is fun.
Yriel is also pretty much the same, though he has 4 wounds now (not sure why... Maybe they just figured some Eldar character should as many other codexes have a W4 hero?). I hever used him much but as Autarchs go he's pretty cool. Fluff-wise though I'd prefer to think of his large blast melee attack as a ninja-like whirlwind of spearwork rather than some lame eye-piece energy blast...
Avatar: The Avatar got monstrously badass, as he should be. Stat increases: WS10, BS10, I10, A5, and W5! I was shocked to see 5 wounds. I was a little disppointed to see his S6 and T6 remain the same as any generic monstrous creature -- seems to me a giant of living molten metal should be T7... I also think I10 is a bit much for such a big guy, but I ain't complaining. I like that the Doom that Wails gives him AP1 attacks, thus the SWORD has more value than just having a shooting attack. I also like that he can be given warrior powers -- they are, after all, HIS aspect warriors. He also saw an understandable increase in point cost.
Phoenix Lords: My fave HQ choices (and I know I'm in the minority there). I won't try to address each one, but in general they all got better! I was afraid they'd suffer from their unpopularity but nope, they actually got cooler. Stats stayed the same, I think, and they retained their Eternal Warrior. Two main disappointments were the lack of changes as reagrds to invuln saves and disciples. Paying 200+ pts for an HQ with no invuln is kind of a ripoff, though save 2+ has more weight in 6th ed than it did in 5th. And no one (other than the Spiritseer) makes their units into Troops -- would have been nice for Disciples to become "makes his/her unit Troops," or at least a scoring unit. I say "unit" cuz the spam potential might have been ridiculous if the whole aspect became troops. They each got more warrior powers to make them more the legends they should be and even the Hawk Lord got cool in this version (and actually does have an invuln). I'm also happy to see they didn't add new Lords, just stuck with the originals (I'm an aspect warrior purist, btw). I'm excited to try each one out, though my faves will always be Karadras, Maugan Ra, and Asurmen (who MUST be your warlord if you take him and gets D3 warlord traits, which is super cool as he is THE original aspect warrior).
I might also note that some folks are coming up with all kinds of creative (and sneaky) ways to combine Independent Characters with different units to produce super-units. If that's your thing then good for you, enjoy that. There's nothing technically wrong with it. I, however, as I said, am an Aspect Warrior Purist (and snob). While combining Nightspear or a Seer with your Reapers or Scorpions might make then super-killy, it pollutes the purity of the kill. My aspects will only ever be lead by their Phoenix Lord--they're too proud for anything else.
If you've managed to read through this whole thing, CONGRATS!
So those are my abridged thoughts on the new HQs, filtered through my personal preferences and play style. What doYOU think???