I think the biggest hurdle the novel had to overcome, and not completely sure it succeeded, is make a story not about the Emperor's Children we already know (Saul Tarvitz first and foremost) and have an empathetic character. The one character who you can stick with predictably dies on Istvaan III, while the others, who are a little jammed in toward the end of the book, like Ferrus Manus, also end tragically.
The duel between Fulgrim and Ferrus Manus was good, in part because I didn't know exactly how it would end, and I was really hoping, hollowly, that the title character would get his comeuppance. While he does suffer a bad fate, it was hard to sympathize with Fulgrim when he had already passed up so many seemingly simple roads to redemption.
At this point in the series, I felt that I needed a break from the Istvaan conflict, so was looking forward to getting to know a Legion uninvolved in the conflict and fairly enigmatic, the Dark Angels, in Book 6, and then moving on to the even more mysterious and strange Alpha Legion, of which I also know very little. Those of you who have read Descent of Angels probably know where this is headed, but I won't spoil it for those who don't.