The book is one of the most moving I've ever read, and so when I heard of the animated adaption, I was excited to see it. However, I found it to be extremely flawed in many ways. It wasn't a complete loss, there were a few very lovely scenes that only made the rest of it look worse.
1. The voice acting. With the exception of Kate Winslet, most of the voices were godawfully annoying. There was no subtlety or reality to these voices, they sound about as convincing as your Saturday Morning Cartoons.
2. The animation. With a few exceptions(I'll touch more on this later), the animation was too bright and kiddy for such a subject, and at times the people look unnatural and sort of disconcerting.
3. The unnecessary song. Not only is it annoying, it's still stuck in my head.
There are several moments that really work, such as the final attack sequence. Many of the backgrounds are very nicely drawn, and there's a great moment involving a crashed plane that I won't spoil for you. The music (other than the aforementioned song) is very nice, and of course Kate Winslet does the best job she can with what she was given.
I think the biggest problem this movie has is that it tries to do to much. It tries way to hard to be funny, exciting, and moving and the only times it succeeds is when it calms down a bit and goes with something simpler. The book is told in very spare language with simple illustrations, and yet packs a much bigger emotional punch.
It's worth seeing once, but read the book.