A good rule of thumb is to watch the movie first, then read the book. Why? Because the book is always better, of course!
If I wanted to prove this point with a single adaptation, I would pick an extreme example like Exodus (book by Leon Uris, film by Otto Preminger).
But in actuality the above statement is not true. The book is better than the movie most of the time, granted. But not always.
Three films come to mind as I consider this topic:
1. The Searchers. John Ford directed what is probably my favorite western movie of all time. It's an epic! Ethan Edwards is larger than life (as are most of the characters, in fact). Memorable scenes full of vivid, beautiful imagery; great, quotable dialog; sharply defined characters; surprising touches of humor; ugly human characteristics on display like vengeance and bigotry, but without heavy-handed artistic commentary to ensure the audience interprets it the way the artist thinks we should. Alan LeMay's book isn't bad, but it certainly is different. The characters are hardly larger-than-life--they are painfully common. This is perhaps one reason the prose narrative is too small to ever achieve epic status. In print, the gruff, vengeful Ethan Edwards is hardly colorful, sympathetic or admirable, as is the screen character which was something of a departure for the Duke to play. It's hard not to admire John Wayne's Ethan Edwards, despite the character's dark, tortured soul.
2. The Man Who Shot Liberty Valance. Hmm. Another John Ford film. Again, characters were larger-than-life in the film version. Even though Ford reversed his stylistic trends (shot in black&white on sound stages, instead of vivid color on majestic locations like Monument Valley) and historic perspective (here he laments the taming of the West instead of celebrating it), he put his unmistakable stamp on this iconic, metaphorical tale. It was based on a forgettable short story by Dorothy M. Johnson. Ransom Stoddard is a character who might serve better as a villain, if he only had a little panache. As a hero or protagonist, it is hard to cheer him on. Of course with Jimmy Stewart cast in the role, even John Wesley Hardin would become a sympathetic character.
3. L.A. Confidential. I lost track of how many times I watched the film. I just had to read the book. When I reached the last page I was still waiting for James Ellroy to start writing something great. After that last page, and to this very day, I am baffled as to how such a spectacular film was inspired by such a lackluster novel. Kudos to director Curtis Hanson for making lemonade out of that lemon, or even detecting something worthy of adaptation within those pages.
Starting with #3 above, I'm sure I already started making enemies, but I'll mention other examples anyway:
The Dirty Dozen (I found the book rather depressing.)
Christine (My opinion--I know not many would agree.)
The Blue Max (Not by much, and the book WAS better in some respects.)
The Rocketeer (I did like both, but Cliff Secord is a lot more likable in the movie.)
The Natural (Even with Robert Redford. What a fantastic flick. What a depressing book.)
I'm sure there must be other examples...
A great book usually suffers when condensed for the screen. Scenes are cut, characters combined, character motivations sometimes lost. But trim the fat from a mediocre work of prose, and sometimes the right director can create a cinematic masterpiece from it.