It's funny, Willow, because I was going to mention Kubrick's The Shining as a film that is far and away better than the book. The miniseries remake, on the other hand, followed the book much more faithfully and was terrible.
For me, one of the the most disappointing screen adaptations of all time was The Thorn Birds. I read the book when I was about 12, and it has always been a favorite. After I finished the book, I rented the miniseries, but I just couldn't make it through. Since then, I've tried watching it several more times, thinking that maybe I was just too immature to "get" the miniseries. Nope, it's simply boring and unwatchable to me. I can't get past the fact that Meggie is supposed to have red hair.