I recently went to see Breaking Dawn 2. (Yes, I happen to be a twilight fan )
I sat through the entire first half of the movie cursing the directors and the horrible job they had done with this movie. It was too fast moving and far too inaccurate. I didn't like it. At all.
Then the second half started and I spent the first few minutes promising myself never to see a movie made by these guys again. But the last 15 minutes of the movie changed my perception entirely.
These guys had done things a little differently from the book, and I really liked that. It came off my black list in a second.
There really is no point watching a movie if you've already read the book. You know what's going to happen in the end. But when movie makers decide to add some twists and turns contrary to the book, it makes seeing these movies worth it.
I know that there are movie critics out there, busy writing negative reviews about how movies often take away the essence of literature, but I love seeing film makers do thing differently, and I wouldn't mind seeing a Harry Potter movie in which Harry's parents are still alive.