I took the kids to see Despereaux last night and we all really, really liked it. First, the animation was just beautiful, incredible. The movie strayed pretty liberally from the book but we weren't too upset by it. The book had a simplicity and was so enchantingly written that it would have been very hard follow it closely for the movie.
I would highly recommend this movie for old and young alike. The discrimination that Despereaux suffered because he was different from all the others really hit home for me. But, it gets resolved successfully and creates a very good, positive message.
Yes, I would buy this movie and watch it several hundred times with the kids. It also made me want to read the book again, just to see how far they strayed.