I think there are cases when an adaptation can be very successful. Take Princess Bride, for example. I love the movie and the book both. I don't know if I would put one above the other, because I like them each for different reasons. And perhaps part of that is because I saw the movie before I read the book.
A case where I actually prefer the movie adaptation to the book is Emma with Gwenneth Paltrow (I don't think I spelled her name right). Again, I saw the movie before I read Jane Austen's book. That could have biased me, because I was absolutely in love with Paltrow's portrayal of Emma, and her relationship with Mr. Knightly. I was quite disappointed when reading the book to find some of my favorite lines from the movie were cut out--until I realized they weren't "cut out," but that the modern adaptation had added them in. Sometimes, adaptation for a modern audience is necessary and can make the work more accessible.
Perhaps the movie ruined the book for me. Paltrow's Emma, even though flawed, was still endearing. I did not get attached to Austen's Emma.