Bring your kleenex! I think if you are a dog lover, you will like this movie, regarless of whether you have read the book. I have read the book (which I highly recommend), but I read it a while ago, so the differences between the book and movie did not bother me much; the spirit was still there. The book (and movie) is about John Grogan as much as it is about Marley, and it is funny and sad and very well done. Many people in the theatre brought little kids, but I don't think they knew how the movie ended. Just expect to cry, and if your children are old enough and like sad movies, then maybe you will want to bring them. If they are little and sobbed for weeks when their goldfish died, you may not want to.
Movie gets two thumbs up, and the book is even better. Highly recommended.