I'm a fan of John Grisham, the books and movies made from them. Loved "How to make an American Quilt" even have the VHS and watch it often.
The worst movie adaption of a book that comes to mind is The First Deadly Sin. The lead was miscast and then to make matters worse, they rewrote the ending. No way would the lead character murder anyone. I was so disgusted at the change I told everyone who is a fan of Laurence Saunders to avoid the movie.