So, I've had a chance to sit and really read the new codex through and digest it for a while, and I have a number of impressions coming away from it.
First off, I enjoy the codex. It's artwork is, overall, quite good. It has a ton of different options when compared to the previous codex, and it has an armory, which is a huge plus for me.
(Subnote on the art: There's two pieces that feature the Heldrake, one looks awesome because it looks nothing like the model (looks waaaaay more dragonesque) and another that looks lame because, well, it looks like the model. Dinobots, Go!)
Kelly did a great job of trying to give people the options to make diverse army builds all themed to certain gods. I'm disappointed there's no special rules for legions like in the 3.5 codex, but this is about the best we could expect from the current style of codices.
Mechanically, many things in the codex seem very high cost/overcosted. This might just be because of the insanely unbalanced point costs we've seen coming from codices by Ward (Or Cruddace's IG), but I do feel like many of the units are not that competitively costed for their capabilities. The heldrake in particular seems about 30 points over what it should be, mainly due to it's lackluster offense, especially when you look at vendetta gunships and the like.
Many of the new units seem to virtually lack any options, which is irksome to me. This is so heavily contrasted compared to other units that it's even more glaring. I feel like things like the fiends and heldrake really should have had more customizability.
Some of the icons are terribly overpriced for what they do. The Tzeentch and Nurgle ones are the most guilty of this. When you run the math with both of them, they're simply not worth the cost, in my opinion.
The demon princes are a bit of a head scratcher for me. They lack Eternal Warrior, have less leadership than what one would expect, and don't benefit from marks in the same way. They also get VERY expensive very quickly. Also, weirdest of all, they don't get to roll on the boon table when they kill enemy characters. I still plan on using one in my army, but that's more due to a thematic idea than anything else.
One thing that surprised me is the lack of solid anti-flyer options in the codex. Havoks can get flack missiles, but the math on them is hardly advantageous given the cost of the missile launcher and the upgrade. The Heldrake simply gets outclassed in flyer v. flyer combat by many things 50 points cheaper. I guess I just expected that, with how powerful, omnipresent, and cheap many flyers are in the game, that 6th edition codices would try and bring them better in line. A bit disappointing, but manageable. (My main is currently Tau, and it's not like they are overwhelmed with all their access to skyfire.)
Also, I'm a little surprised that they didn't really play up the allying with demons angle in this codex. There's no access to teleport homers or anything focused on synergizing with demons (overtly, anyhow).
Overall, I'm enjoying the codex, although I'm a little worried about the high costs of many of the units when compared to what they can do, and when compared to the current game context.