Bridge to Terabithia is one of those books that gets into your soul and sticks there for a while. It's also one of those books that means different things as you grow older, and as you grow into the lessons that it has to teach.
It's about friendship, and family, and the death of loved ones, and the ongoing death of being unseen by the people that you need most to see you. Paterson manages to capture all of these things both at the depth of adult commentary and at the depth of childlike experience, resulting in a book that is wonderful no matter how old you are.
The recent movie was both wonderful and disappointing. Imagination and the escape to a fantasy place is a major part of the book, but by making it so explicit in the movie, much of the power of imagination is stolen from the viewer. If you see the movie before you read the book, you're dong yourself a great disservice. Of course, that's true of any book. :-)
As usual, I have a word of advice for parents. This book is very, very sad, but it's also all about redemption. Read it before you read it to your kids, but do read it to your kids. It's worth the effort, and it deals with the issue of death of a friend in a very redemptive way.