I got A Thousand Sons and read it the other night (as in read the whole thing after I got home from work) - I would say calling it the best Horus Heresy book thus far isn't too much of a stretch. I honestly wasn't too sure about this as McNiell's Ultramarines, while good, weren't great (tending towards either bland or cartoonish characterisation [replace your mental picture of the Dark Eldar dude in Nightbringer with Skeletor, it won't matter much]), and I had reservations about the fall of Magnus being portrayed as simplistically as Horus'.
But all that was pushed aside and we got some great, well fleshed out characters, a plot that made the character's actions make sense and a lot of subtle hints.
Some of the thing's I noticed (some minor spoilers):
Magnus calling Russ' barbarian personality a mask.
The portrayal of Russ as a frothing madman has always bothered me. In the Rogue Trader books there were all these little Sun Tzu like quotes attributed to Russ, so having him running around crazy didn't really go with that. I like the idea that he puts on the wolfman persona as that's what is expected of him.
How much does the Emperor know of what's going to happen?
Magnus and Ahriman both hint at the Emperor seeing the future (mentioned in the Legion's name discussion amongst others), so why doesn't he act on it?
Perhaps the plan is to turn the galaxy into the "modern" 40k 'verse (meaning the vision the daemon showed Horus was the truth of the Emperor's plan). By that I mean the Emperor saw that humanity's only chance of survival was for him to supress every other religion, then set up a hash, brutal dictatorship worshiping a tangible 'god' to ensure the mistakes of the Dark Age weren't repeated (ie ripping off Dune again).
Maybe he saw the Heresy, but thought he could defeat/turn Horus back, and deal the Chaos powers a major blow by robbing them of the champion they invested so heavily in - but when he had to confront him in the flesh he lost his nerve (bringing some much needed humanity to the Emperor's character)?
This of course contradicts the existing account where the Emperor only destroys Horus when he sees how truly evil he's become, but it could be worked in.
But regardless, the book is top notch - the dialogue is well done, the characters aren't annoying, Magnus is a fool - but not an idiot (like Horus), Ahriman tries to do something when he can see rot setting in (unlike Loken), the battles are fast paced and some of the best in the series, and lastly - the remebramancers aren't annoying and whiny (and they tell people about the improtant information only they know in a timely fashion!).
The only letdowns are that I spotted a few typos, and the very last section felt a little rushed and could have done with another page or two.