This post is a little bit overdue, but up until recently, I haven't had a good chance to sit down with the codex and try to slam out a list. Now that I have, I have some general ideas about the book.
First of all, the change concerning the Legion Marines into Elites, which are unlocked by the Marked HQs, is interesting in several ways. While in terms of points it is not terribly burdensome, it does make some level of impact. For example, a list that wants to say infiltrate Khorne Berserkers will need, presumably Huron and a cheap Khorne Lord. It is a potentially awesome combination, but it still requires a bit of investment and planning in terms of list construction.
Somewhat linked to this topic, the Special Characters in the codex are pretty awesome. Lucius (who is one of my personal favorite characters in the 40k universe) still sucks ass, but the others got some definitely interesting bonuses. The fact that some characters have an inbuilt Warlord trait makes for some crazy combinations. I have heard some complaints that there are no new characters, but frankly, that doesn't really bother me at all. They got buffed overall, and that's the cool thing.
Another thing I have noticed is the overall decrease in points across the codex. Many options are cheaper, and they have a greater variety overall. The fact that Sergeants can all take combi-weapons and other cool options makes them all pretty neat. I think the codex has improved simply based on those point reductions.
Along with these benefits, however, there is one glaring issue I have with the codex: Champion of Chaos. I really HATE mandatory anything. I find it burdensome, and knowing my luck, it would be the reason I would lose a game. In addition, your Champion can become a Spawn or something else pretty stupid. Daemon Prince on a Champion? Awesome. Daemon Prince from a Lord? NO. Losing all equipment is kinda terrible. If I needed an HQ Daemon Prince, I would have just bought one myself.
Overall, it's a very fun codex that has some great benefits. Plenty of vehicles, flyers, and some anti-flyer built into the codex. Any fan of the Chaos Marines will definitely bring them back out onto the table.