sometimes the screenplay compliments but it does much much hurt to the book itself. Its a nice feeling to have personified version of the characters you like to read but the plot itself gets almost inorganic, so much has to be deducted and adjusted in order to cope with the time limits etc.
yea screenplay is key, i suppose they can't follow the book scene by scene and they have to modify it to make it practical...but i refuse to let go of the pictures i make in my mind:D
personally i would always opt for the book rather than the film adaptation. it requires more use of your imagination, its always far more detailed. films somehow very rarely manage to capture or retain all of a book or its spirit. sometimes the film makes changes to the ending. i usually find myself disappointed by the film lol.
The main difference is that in literature we are inside the the characters' heads and privy to their thought processes, and that is a dimension which film does not achieve; or at least not in the same intimate way that words do.
There are some godawful adaptations. The good ones include; To Kill a Mockingbird, Breakfast at Tiffany's, Remains of the Day, and Straw Dogs. In fact, Straw Dogs was way more powerful than the book.