Never judge a book by its movie.
-- J. W. Eagan
My life is full of coincidences these days. I was *just* discussing this subject with a friend. She quoted something from Peter Pan, about Peter having nightmares, a surprisingly deep scene which I'd never heard of, and it didn't fit at all with the Peter Pan I knew from Disney.
Similarly, if you have only ever seen James Bond movies and not read any of the books, you'll think it's perhaps very entertaining, but also quite superficial fluff action stories. But in the books it's actually quite different, there's sometimes a lot of pathos. At the end of one book he finally gets married, and his wife is gunned down while she's still in her wedding dress. I'm not sure I've seen all of them, but I don't think such a scene ever made it to a movie!
An of course an egregious example: Battlefield Earth. The movie was just stunningly bad. They had changed everything. Mostly because technology and the budget was not up to creating very realistic ten feet tall aliens descended from wolves, the aliens did not look alien at all, and many other things. It was basically laughable. And it was a claustrophobic-feeling action movie, while one of the things I like best about the book (a very large book, for years one of my favorites) was the many stunning vistas that the author created, he had a unique talent for creating a feeling of space. (Not outter space, but mountain landscapes or big forests, etc.)
Of course they are very different media, and often one just has to tell a different story. But it can be done well. For example I think the Harry Potter movies were perfectly adapted, though of course the stories had to be abbreviated, they had the exact feel of the books.