Me too, with Garp. I liked the movie, but thought the book was better.
I often watch movies of books I read, and less often read books the movies I saw were based on.
I love the LOTR movies, but I never got through Tolkien. Time and again, I think I should give it another try, but there are so many books in my shelf.
And I knew the first three Harry Potter movies before I started to read the books. It's funny how the movies manage to capture me in a different way than the books. I actually consider them two different stories - or two different versions of the same story (which they are), like an image painted by two different artists. You'd still consider them separate, wouldn't you? Well, I do ... and it's often like that for me with books and movies. Because usually they're not that close to one another, and I try to see the movie as a separate piece of art - or, at least, a separate work.