I can't talk about books I LOVE, but about books I LIKE. "Like", not "love". For example, I had this stupid obsession with Twilight but I wasn't excited about the movie, I knew it was going to be as horrible as the book was; and my obsession was more about Edward Cullen than the story itself.
I read the LOTR books after watching the movies. My opinion on this is not the same as someone who read them first but I think they're the best adaptations that could have been made, and the changes that Peter Jackson and the other script writers made were for better (I love what they did with Arwen's character and the story of Arwen and Aragorn, so when I read the book I was a little disappointed because Arwen wasn't there as much as in the movies)
About Harry Potter&The Half-Blood Prince: I liked the movie a lot. What I expected to see was an entertaining movie and that was what I got, although they almost "raped" the story to make it into an "American Pie goes Hogwarts", but I laughed a lot so I forgave them for their mistakes. This movie is one I'd say it really lived up to my expectations. I knew it was going to be great and it was.
I remember reading Eragon, by Christopher Paolini, and I really loved it and then I saw the movie and it was a big "WHAT THE HELL??" because they completely messed up with the story in such a way it was impossible to fix it. It's not only that they changed things from the book, I'm used to that (LoL, Harry Potter), it's also that the actors performances were BAD and the special effects not very good. I think that, in a weird way, the movie adaptation of Eragon is even a worse movie than Twilight.