I really enjoyed this book the first time. I absolutely loved it this time. Incredible uses of psyker powers. Far and away the best I've read so far. The variety and inventiveness are amazing. And not remotely just flames, telekinesis, and all that flashy, forceful stuff. I'm talking about very subtle things, and even just his perceptions of particular ideas and moments.
Ahriman's power is as staggering as it should be. I'm not positive he sleeps in the entire book. Probably. Heh. Yet he has to go from one insanely dangerous situation to another, maintaining his calm and focus, planning impossibly intricate things out to the tiniest detail, and finding immense reserves of power.
The story is great, with some very interesting characters.
Eisenhorn and Ravenor have been my favorite BL books since I read them, which was in the very beginning of my BL introduction. (Right after the Blood Angels Omnibus.) Ahriman: Exile is most definitely tied with them for the top spot.