I'm a professional academic, and my specialty is film. Within my specialty, I've actually done a lot of work dealing with film adaptations from literary sources (including some published work). With my qualifications out of the way, I think the one thing I've learned about the relationship of films and books is that the quality of either is not dependant on the source material, but on the ability of the artist. A book is not a movie. No movie should ever, ever, EVER be exactly the same as the book, because there's just too much difference between the media for that to result in a good film (a potential exemption is a film based on a comic book, because unlike a traditional novel comic books already incorporate visual elements). In an ideal situation, if a good novel is turned into a film the director should be of equal talent to the author, and should know when to retain elements of the novel, when to eliminate elements, and when, if it is necessary, to actually make changes to the novel in order to create a valid work of film. So in answer to the question, it depends on the book or the movie, and hopefully the answer is both, but in different ways. For some of my favorite examples, see the Laurence Olivier Hamlet or a Japanese film called Double Suicide, based off a Chikamatsu play.
I completely forgot about "Celestial Navagation." Some of the best lines ever. Toby: How do you know we're going east? Sam: The sun rises in the east. Toby: It's dark outside. Sam: Also, that bright star in the northern sky is Polaris.