I have to recommend not spoiling this book before you read it. Don't see the movie, just read the book. It's a mystery and the fun in reading it came from trying to figure out what was going on.
The beginning is a little slow. There's lots of backstory, and a lot of information to set the scene. There were things to keep me interested, things I noticed that I wondered about, but it wasn't until the half-way point that I couldn't put this down. Everything in the first half of this book it building to a serious turning point in the middle. Once I reached that part, everything changed.
I didn't like Amy at first because she sounded pretentious. You hear her side of the story, and get flashbacks of her and Nick's past through diary entries. As the first half of the book went on I started to like her and I started to believe that Nick had killed her. By the end, I was rooting for Nick.
The ending was satisfactory, but not entirely satisfying. <Spoiler> I really really wanted Nick to kill Amy in the end. It felt like she deserved it, but doing so wouldn't have given him a happy ending. There was a part of me that rooted for Amy at times, and there were even parts that wished Nick and Amy could have what they used to -- so a part of me was very satisfied with the ending, but at the same time, I know its all fake and so it's not really. </Spoiler> It's a very twisted book, but I loved it.