I can't say I was surprised by the ending, or rather what happened at the end, but I was kind of irritated with it because I wanted this book to be different, and not do what I figured it was going to do from the ending of Delirium. If you've read the books this should make sense. 
 One of the best things about Pandemonium was that it wasn't the typical less than exciting middle book in a trilogy, nor was it the same formula as Delirium. 
 The Then chapters were very informative I liked the Now chapters better. There were so many things going on and so many hints on what might happen in the future. There were so many eye opening things. 
 I liked Lena a lot more in Pandemonium. She lost the naivety that she had in Delirium and I loved the Then chapters where she was gaining her strength (physical and mental). And her planning and strategy in the Now chapters. 
 Lena thought about Alex a lot. And it wasn't just "oh I miss him" "what will I do without him" though there was a bit of that, but she understood in her head that he would never be coming back to her and made herself accept it. She made a lot of decisions based on what Alex would have/had done, especially in regards to Julian. A lot of the explanations she gave Julian were very similar to how Alex explained things to her. 
 I really, really liked Julian. He was completely different from Alex, but he was a strong character, and even though he was raised like Lena he had some pretty good intuition and street smarts that he didn't handicap her. 
 One of the things that was a bit different in this book (compared to other trilogies/series) was that all the characters except for Lena were new. I liked Raven and Tack (I loved their bantering!) even though I didn't agree 100% with the way they did things. They were pretty well balanced and did what they needed to do, even if it wasn't always popular. Raven's backstory was one of my favorites.