I started reading this expecting it to be your average run of the mill 40k fiction, having read a few of said books before; I wasn't really THAT much into the whole Inquisition thing - initially I bought it merely because I wanted a hefty book that would take me some time to finish.
Having said that, this now ranks as my no. 2 most fave book of all time (and I like to think I've read quite a few). The whole thing just drags you right in and you have to physically force yourself to stop turning the pages; right from the start it's pretty action packed all the way to the end. My only beef with it is that some of the characters come and go, and it would have been nice to see some of them develop a bit more (I'm a big fan of Midas) before their exit.
Even so, you do get engrossed in the characters (although you still somehow feel that you are kept at arm's length from the main characters emotions, but you come to realise that this is more due to the pretty emotionless state that he exists in anyway), and when they do suffer some injustice or other it really grates on you.
It's not a pretty story, especially towards the end, but then again if you wanted a book without plenty of death, you wouldn't be coming near a 40k novel ;)
WELL worth the read. One of the few books I've read twice in less than six months.
On a similar note, the Ravenor novels are of a similar standard; not *quite* as good as Eisenhorn, but not far off.