A few years ago, I took a class on Children's Literature and Film because I was interested in how the two mediums connect.
Before that course, I was of the opinion that the worth of a film based on a book was solely determined by how accurately it mirrored the book. If it deviated more than the slightest bit, changed characters or plot points, it was BAD.
Ok, now some movies based on books do turn out badly (I still am so disappointed by The Lightning Thief) but I've changed my criteria for judging. Does the film honor the spirit of the book? Are the changes made necessary because film is a visual medium and elements need to be handled differently? It makes a difference. I have to admit, I loved the Harry Potter movies and they weren't always faithful to the text of the books. Plot points changed and, in my opinion, some of the best characters never even made it into the films (sorry Peeves).