Really? Sometimes when I do that, I end up HATING the movie bc it is so sub-par to the book. Bourne Identitiy is the perfect example. Awesome book - heck awesome series, but the movie just didn't cut it.
Good fight scenes, but the plot was a bit off and didn't really cover the most important and interesting parts of the book. 
I agree -- there have been times when the movie has just been ruined for me because it doesn't meet my expectations of the book, but at the same time, in this case, the book's prose gets you so into the themes and the characters of the book, it just doesn't matter.