Many times, the movie is not as good as the book. Some times, the movie is entirely different than the book and can stand on its own - The Wizard of Oz is a great example. Some times, the movie is the equal of the book (the Colin Firth P&P, for example).
And then there are those very rare times when the movie is far, far better than the book. I've recently experienced this with our afterschool book club. We read Joan Lindsay's A Picnic at Hanging Rock as a group. The kids liked it, even though they sometimes had difficulty understanding and following the slow pace of the book. The mystery of what happened to the girls intrigued them. Then we watched the movie.
The setting, the music, the pacing: it was perfect. The kids were loathe to stop the film at the end of the period, and just as loathe to start again the next time because it was so eerie, so creepy.
When it was all over, they were unanimous. The movie was far, far better than the book. How often can you say that?