This book was so close to being a five star for me... so close. Overall, I thought the concept was original and fun, the plot kept me very involved, and the twists were well-executed and thoroughly satisfying. It's rare to see (or least suspect) a twist coming, but still be so thrilled about it when it comes to pass, and that's how I felt reading Pandemonium. 
 (view spoiler)[The primary thing that took this from five stars down to four for me was the random, somewhat unnecessary, and completely un-sexy sex scene which was clearly non-consensual (someone who is sleeping cannot consent, even if upon waking they are interested) but was never treated as such in the book. (hide spoiler)] 
 Daryl Gregory is fast becoming one of my authors to follow (having read two of his books now and almost thoroughly enjoying both) - his ability to twist together different genres into something new and delightful makes his books intensely readable, and I'm interested to read more of his work.