I don't usually read sci-fi, but I though the premise sounded interesting enough to try. I liked the diary-form of the narrative, and that it is a YA not focusing on instalove or relationships at all, really, just a girl's pretty pragmatic approach to surviving in a really messed up situation.
My biggest beef with this book was the pacing. It swung between boring and interesting, and I found parts pretty repetetive. Particularly monster hunting through spaces. Could have done with less of that. I also thought there were way too many secondary characters to try to remember, due to all the Sentari squads. I cared about very few of them as you never get a feel for who they are (it's Cass' diary after all, and so she's not all that interested in doing character analysis). Most of the time I could only remember the captains.