I realize this is a very old post, but still a good one, considering the Heresy books are still being published.
Since the last posting, there has been Descent of Angels, Legion, and now Battle For the Abyss.
About the only thing I can say is that the series seems to be taking a down turn, or more particular, not answering questions the way I thought they would.
An older book, Eisenstein, seemed to have ended with the Imperial Fists returning to Earth and warning the Emperor, while at the same time sending an armed delegation (the first Emperor's Chump Sigismund) to find out what Horus is doing. There was a paragraph or two that described all-out war broke out amongst all the planets, but that was all the detail included. So many things left unsaid. I mean, what happened when Sigismund went to the Istvan system? What happened when Rogal Dorn went back to Earth? There's a whole story in itself: "After Istvan".
Legion, in particular, had a HORRIBLE ending, if you ask me. I won't spoil anything (even though the book's been out for a long time), but the excuse given for them going over to Chaos made absolutely no sense at all. Fulgrim did the best job of describing a legion falling to Chaos, whereas Legion just simply happens.

Doing what they did, for the reasons they gave, did nothing to explain how they went from Emperor-loving to Daemon-worshipping.
Now that I'm reading Battle For the Abyss (about 100 pages in), I'm still left mystified as to the REAL story of how Erebus and the legion fell to Chaos first. If I didn't research fluff online or ask other people, I never would've known the Word Bearers were the first to fall, then set the Heresy in motion. If anything, their story should've been first, told in the same manner as Fulgrim, then ended with Erebus actually setting the wheels in motion to "poison" Horus. The Battle For The Abyss should've been a follow-on story, because it jumps right into the story as the UltraMarines are about to be bitch-slapped in the opening stages of the Heresy. No explanation at all of the events leading up to Abyss, other than fluff you have to find elsewhere.
I haven't heard too many good things about Abyss, although I still went out and bought it. I'm tied to the series and like this genre, so I'll put up with it, but if I had one complaint, it would be about the "timeline" and priority of all these books. They should've come out in a better order, to tell the story of each Legion involved, and bring them all to the point of ambush at Istvan V. Instead, they keep hopping backwards and forwards, or leaving chunks of vital story-telling out.
Long thread, I know, but there was some catching up to do, and I find myself a little bored at the moment.