Similarities and differences on book-based movies

I always believe that books are much better than the book-based movies, because in most of de cases the directors change things that in the books are very important, or add things that in the book don’t appear. So, for that reason I prefer books.
But almost think that when people only see book-based movies and they like it, it’s no necessary that they read the books before, because there are a lot of people that don’t like reading and I respect that choice. But in some case, for example in school times, when was a reading test, I thought that was better read the book before a movie because in the books were details that sometimes didn’t appears in movies and that could be bad at the moment that someone do a reading test.
I think that for the esthetic and the effect of the movies it’s no necessary change things of the books, because that doesn’t has influence in the movie, I prefer watch I book-based movie more similar to the book than an spectacular movie with a lot of amazing effect.
I believe that when someone are a very fan of an specific books and then watch the movie, most of the time the movies doesn’t satisfy the expectations. For example, I was a harry potter’s books fan and I don’t like very much the movies, but like the word saying the movies are only “bases on books”, nothing more.