Usually, when I see a movie/movies based on a book, I think the movie is worse. This is almost always the case. The Harry Potter movies are no exception. (I have so far watched the first six movies.)
However, I noticed something unusual about these adaptions. Although the movies were certainly not bad, and I did enjoy watching them, watching them felt considerably longer than reading them. They just dragged on… and on… and on… until I wondered how much longer they could possibly go. Now, the books took me more than two hours each to read, and the movies are about two, two and a half hours long each, and therefore had to pack more action into less time. Why were they so much more boring?
I think that it may be because of all the small subplots which Rowling put in the books, possibly to keep the interest revved up while the main plot was brewing: for example, the slight Harry felt when Hermione and Ron were made prefects in Book 5. However, most adaptions of books leave out minor strands of their literary originals without becoming hideously dull in comparison. (I note here the Lord of the Rings movies, which, though hardly perfect, remained good adaptions and excellent entertainment even without Tom Bombadil, Ghan-buri-Ghan, the Scouring of the Shire, and so on.)
I do wish to make one exception to this rule in the movies I have so far seen: Prisoner of Azkaban was excellent. Why this is, I cannot say, other than that whatever the differences were that made it more entertaining, they probably stemmed from the director, who for some reason directed only that one. (Why? Why? And for that matter, why did Peter Jackson take over the Hobbit
movie movies from Guillermo del Toro? WHY?! I ask fate!)