I think that it really depends on the particular book/movie. For instance, I really like Lord of the Rings movies and the books, while I think that the Harry Potter books are slightly better than the movies, especially the more recent ones (don't let me start about how the last director BUTCHERED the Order of the Phoenix!). I haven't really read most of the other books-to-movies that my friends have very strong opinions of, but from talking to others I've found that it depends a lot on which you are exposed to first. I personally think that I would like the Narnia movies better than the books if I ever read them, but I have friends who hate how different the movies are from the books. Some changes to the books are permissible in the movies, simply because it's necessary to make a reasonably short film, but changes on the order of those in the last HP movie are, to me, unforgivable. The changes in the LotR movies were done in such a way that they weren't really noticeable unless you had memorized the books, with only a couple exceptions, and I think that the changes in the book scripts added to those movies rather than detracting from them. For a specific example from those movies, the new Warg scene in the Two Towers enhanced the personal relationships between the characters and added a good action scene in the middle of what would otherwise have been a rather slow-paced section of the movie.