The "Blood Wars" books were my first exposure to the Planescape scenario, which I think rocks. Too bad "Blood Hostages", the first book of the trilogy, didn't get close to the potential of the scenario at all.
This second one, however, is captivating. It has been the first book in a while that I read in one sitting, dropping into bed at half past two. The story is compelling, and the characters are drawn much better than in "Blood Hostages". I agree with the other reviewers that it's especially the mad Nina, a powerful female character like you don't encounter in the Fantasy literature very often, that makes the book stand out. Finally, without giving anything away, the end of the book is brilliant and really evil, and I can't wait to read the final book so the characters can finally get out of this ugly situation. (I hope they will, at any rate.)
The reason why I'm not giving the book five stars is that it lets opportunities pass. After a third of the book, we are told about a special and unique power of the two main characters; but while one might think it should be, this power is never a driving force behind the storyline, it just comes in handy in some situations.
About the first book: I'd recommend to buy and read it before reading "Abyssal Warriors", even if it's substandard. You'll be much more able to appreciate this one afterwards (and not just because of the contrast in quality).