I'm not sure you can expect much from a film of a book other than that it is a hugely enjoyable spectacle - after all, that's the only thing the film can give you that the book itself doesn't.
However, I can't help but feel I need to urgently re-read all the books before the film starts being made and the publicity comes out - once you are being bombarded with Hollywood's view of what it all looks like its all too easy to forget about your own. I re-read the lord of the rings before I saw th first film and I can't help but feel it helped me to retain my own picture of the characters etc in my own imagination.
I'm sure Holywood sees it as a chance to make another LOTR, in fact, they could stick to just the first book in the hopes that they can make another hugely lucrative franchise that doesn't have to stop after film 3...
I definitely enjoy watching films like these but I'm not 100% percent sure they don't take something away from the original book - I definitely advocate re-aquainting oneself with the text before bombarding your senses with the full dolby-surround cinema experience....