I really liked Pandemonium. I definitely liked it more than Delirium, and you can't help but wonder what's going to happen next. You're thinking that Alex has been dead for the entire novel, only to have him return at the very last minute.
I really like that each chapter is a different time. Seriously, the best way to describe it is past-Lena and present-Lena. It's nice to see the difference in time, and what happened while she was in the wild, and how she infiltrated the DFA. I liked that we learn having the surgery is all about control, which should have been obvious to me. Oliver did a really good job of keeping things going, and the book doesn't feel like filler for the next book.
I really didn't like the ending, where Alex appears. I get why we think he's dead, because it really seemed that way. And I suppose that Oliver needed to get a romance in there somehow. For me, it was a lame attempt at getting in a love triangle- almost like Oliver couldn't think of anything else.
Overall, it gets a 4 out of 5. It was fast-paced and enjoyable, but the ending and the romance felt a little forced.