Popular Books That Have Become Movies

I found this link to a list of books that have become movies

http://www.goodreads.com/shelf/show/books-that-became-movies

Numerous times I've run out and caught a movie because it was based off a book I just read. Usually I end up being a little disappointed because the movie doesn't always match the picture I've painted in my head. Some books I've read prior to seeing the movie are: One For The Money, The Hunger Games, Beaches, New Moon,Breaking Dawn, Eclipse, Flowers In The Attic, Eat, Pray, Love.

On the flip side I can only think of a few occasions where I've read a book after seeing a movie. For example I did start reading the Twilight series after seeing the 1st movie.

Which do you prefer reading first or watching the movie first?