Wow. I really enjoyed this book. Who can't feel bad for Cinder? She has been put down her entire life (at least that she can remember) and her self esteem isn't great. She's an incredible mechanic and catches the eye of a prince. That was the only weak spot that I detected in the story. Cinder and Prince Kai fall in love in a very short time. Short enough that I would call it insta-love if it didn't feel genuine, but it actually does--at least more than in the other stories that I have used that definition, and it's certainly better then a lot of Cinderella stories. Cinder and Kai clearly care about each other, but their love story isn't as corny/stupid as most of the books in the YA genre (at least so far, hopefully it doesn't get worse in later books,) though it still can't beat Ella and Char from Ella Enchanted.
I was disappointed (and sad) that Peony died, but I guess Cinder wouldn't have been able to make it to the ball if Peony had survived. Cinder's reason for going to the ball were complex and selfless, beating even Ella's in Ella Enchanted, which were complex but sort of selfish. This is one of the best Cinderella retellings I've experienced, including both books and films.