Tolkien is a great storyteller. The way he writes hasn't held up as strongly for the ADHD generation, but it's still really really good for anyone that gives it the time to really sink in. It does pick up and, by the end, you will feel exhausted but in a completely satisfied sort of way.
That said, I never had a problem with the film-adaptations. In fact, I like them alot. While they messed up the plot in a couple of areas and complicated it where it didn't need to be, I found it visually to be pretty much what I had in my mind. The exception to this was Legolas, who I always pictued with dark hair, and Faramir, who I just had imagined as completely different. Still, I was more than happy with much of the art-direction, especially the way they handled various locations and the towns/cities.
Harry Potter, idk. The books and movies have come-up side by side, so they do kinda seem to be woven together. I'm not particularly fond of HP, though, so I really didn't care if the movies screwed things about the books up.
David Lynch's Dune is a pretty good example of a book-film gone awry. The weird thing is, Lynch had the atmosphere down. And the changes to the story weren't horrible. But those damn shields...