Raj has just beaten me to it, but watching the BBC Pride and Prejudice in the mid-90s got me reading Jane Austen, but once I've seen something, that then changes how I view the books. If I've enjoyed a book, I'll purposely stay away from the film in case it ruins it for me. My favourite Austen book is Mansfield Park, but I don't want to see the film (I heard they changed the main character to make her more outgoing, to appeal to a contemporary audience. An outgoing Fanny Price is really not the point of Mansfield Park).
As someone has said earlier, with complex books, it can help having seen the film/show. I enjoyed reading The Hobbit when younger, then tried reading The Lord of the Rings. I couldn't keep all of the names straight (kept getting Saruman and Sauron confused) but after seeing the film, I went back and reread it and found it much easier. Also agree about Les Mis, saw the show, then read the book. Actually, that is one thing where I can keep the characters separate, maybe it's the depth of the book. I love both the book and the show, but see them as separate things.
Some stories you think you know, turn out very different when you read the book. I'm thinking of Peter Pan, but after having read The Hunchback of Notre Dame, couldn't believe that Disney had turned it into a cartoon.
Oh, I saw the film The Prestige, didn't realise it was based on a book, then saw it in the library, so borrowed it. The film was good, but disturbing. The book, on the surface was quite dissimilar, but was very disturbing.
By the way, I'm 35.