The movie definitely differed a lot from the book (don't worry, no spoilers) - certain characters were changed, a couple were left out, a few sub-plots never made it to the big screen and the overall story works itself out differently in the movie than in the novel. After the movie, Dave asked me which version I liked better and my honest answer was that I really loved and appreciated both. One thing that I applaud Cassavetes for was that he used a good amount of the author's voice and dialogue in the movie (he did this with The Notebook too). Picoult is definitely present in the movie, which made me like it all that much better.
Overall recommendation: Definitely worth the $10 to see it in the theater.