When changing from book to screenplay, some things must be left out or even changed. But this movie's changes are inexplicable. The movie opts out of everything that made the book great, from the author's unique sense of humor, to the slight tension of romantic interest, to the protagonist's (and everyone else's) frustration at not knowing who he is. The movie just gives things away too fast. Major characters and essential elements of the plot are just tossed out, leaving the movie so different from the book as to be almost unrecognizable.
They did do a good job with the special effects. If you don't want to bother with the story, but just want to see some cool creatures and stuff, then I suppose this movie is for you. I expected more.