There are so many reviews of this book, I'll let the longer, more detailed ones speak for themselves as to the mechanics and character development. I'm going to stick to my feelings about it.
I think this may have been one of the best books I've read in a long time. I was an emotional wreck at the end of it. I wish I could rewrite the ending - I wanted A to somehow wake up in Alex's body, and have Alex be the amazing person to give up his life so A could have one. . But then again, it wouldn't be the story it is without the ending it has.
I found so much of this book just profound in its observations of the truths in life. That's what makes it so good; it's not just a story about a boy and a girl, it doesn't just have a twist -- it is a complete commentary on the human condition. And especially so the teenage human condition.
When any kind of a writer - prose, music, stories, etc - can put his reader's feelings into such concise words, as if they've read our minds, it is genius. It moves me. This book moved me. It will move you.