Just to join in on the LotR discussion, having read the books (*after* the films, admittedly), I have to say that I find almost all of the changes very much justified. Not all things that are great in a book work in a film. And also some of Tolkien's style (in particular, dialogues) is actually occasionally a bit disappointing, and Jackson really improved on that.
What Tolkien was really amazing at is crafting this world with this incredible amount of detail - and Jackson put a *lot* of effort into being true to that vision.