My favourite character in the book was probably Ami, even though she wasn't a main character. She was independent, and really nice. I also liked Dr Harriet, because she was nice, willing, not brainwashed, and she taught Pia how to be free.
Pia was a goo heroine in all, but she was a bit stuck-up (and being told she was perfect all the time didn't help). Like, when she first met Eio, she kept feeling the need to remind him that she was perfect. Although she did get more accepting as the book progressed. I didn't really feel all that much for Eio. I mean, I'm glad that the romance isn't overwhelming the book, but we didn't really see much of him, he was just "that cute guy" for a lot of the book, and I didn't see the point of some of his choices.
I also think that Jessica needs more description. I mean, all I can clearly picture is the field of elysias, and the fence. There was barely any description of what Little Cam looked like, and although I appreciate authors not having two-or three page descriptions describing every inch of the room (*cough*Eragon*cough*), I would have liked at least SOMETHING telling us what we were supposed to be imagining.
I was a bit unsatisfied with the ending. (SPOILER ALERT) Is Pia going to die or live forever, just being able to bleed? What happened to Dr Harriet after that? Did Pia ever get to see the outside world? (END SPOILER) and so on. So many questions...
Other than those things, the action and the plot was perfect. I was just swept up into this story. I definitely recommend it.