There are three types of book-to-film adaptation:
1) Good adaptation, bad film
The film sticks to the book's plot, characters and themes but forgets to be a great cinematic experience while doing so.
Examples: Perfume, Cloud Atlas, The Lovely Bones
2) Bad adaptation, good film
Book is filleted and sometimes jettisoned for the sake of the film. In some cases, only the title remains. Not necessarily a bad thing.
Examples: Do Androids Dream of Electric Sheep?, The Shining, any James Bond novel except for Casino Royale
3) Good adaptation and good film
The best outcome for fans, studios and movie audiences. Rarer than you think.
Examples: Life of Pi, The Lord of the Rings trilogy, Atonement