I like to enjoy both movies and books, so I prefer to watch the movie first, because of changes made. Then, if I want to read the book later, it fills in the gaps and explains things better and I still enjoy both. But if I read the book first, I can be a really bad nitpicker like a lot of HP fans I know. ;) Which is why I'm worried I won't like the movies for 6 and 7 as much since I've read all the books now. :-\