I have had that with a couple of Stephen King ones I have read all of his books and some of the films that are adapted from his books aren't very good to be honest! The green Mile was a brilliant (I think) movie as was Misery to name just 2. The Tommyknockers and Salems' Lot were terrible IMO! I am quite suspicious when it comes to movies being made when you know that the book is good! But I will usually read a book first before I see it on dvd or anything.....I know what you mean though, it can be very disappointing when the book is great but the movie is rubbish!
When I was younger I read The Exorcist (which I thought was a good read) and when I finally got round to seeing the movie I thought it was the biggest load of rubbish! But that's just my opinion on it, I know a few people thought the film was very scary.....I just didn't get it!!