It's funny, but with The Lord of the Rings movies, I felt like having read the books added to the enjoyment of the movies, but with The Hobbit, I'm starting to feel like it almost is a detriment, given how very different many aspects have become, and how much has been added ("shoehorned in" is the term I would use, but that's just me). Reading the book doesn't give you much of an idea of what to expect from the movie, it seems like, except in a few select scenes that have been more or less faithful. Not saying the first movie was a bad movie, or that the next looks to be either. But they fail as an adaptation of The Hobbit, IMHO, and I can't help but feel that I would enjoy them more had I never read the book they are supposed to be based on.
Maybe it has to do with the fact that I never had any illusions that The Lord of the Rings could be filmed without making changes, and a lot the changes made seemed like they were done to allow such an expansive story to fit within the running time. Whereas with The Hobbit, I've no doubt that a massive trilogy COULD easily portray the source material (and in fact is too MUCH runtime for it) but the changes made seem to have been more for the sake of squeezing some more action scenes into the narrative, and tieing it in more strongly to LotR, neither of which seems at all necessary for a Hobbit film, IMHO.