SubscribeMichael Stackpole has refered to the relationship between authors and readers as an ongoing game of Name That Tune. That the reader is constantly thinking, "Oh, I know how this ends," and it is the author's responsibility to say "No, you don't." If the author is right, and the reader gets to the end thinking, "I see how we got here, and it makes perfect sense, but I never saw it coming," the author has succeeded. If the reader was right, they're unlikely to ever read that author again.
I'm inclined to think that Michael is right, at least most of the time. I know it's been true for the books I have enjoyed the most. There have been a couple of cases of books that were very predictable that I enjoyed immensely, but they have tended toward cliché books which are a literary equivalent of B-movies. Immensely fun, but not high art.