Not as bad as some on this site say. I am a huge Terry Pratchett fan, I have read all the Discworld books and few of his other books. While this movie/TV special doesn't really live up to the books, it isn't half bad. I enjoyed revisiting the story with some pretty snazzy special effects- even if some of the subtle humor was lost (or was it just very very subtle?) I have to admit that many of the jokes in the books I probably miss. I know sometimes it takes 10 pages before I realize he's just made a joke, which makes me feel stupid, and then I laugh at myself for being stupid. Sometimes I don't even catch them till the second reading! This movie/TV show (or whatever it is) doesn't really have that feeling.
If you are a fan of Terry, i say you should watch it, if your not.. then go buy one of his books and read that before deciding to watch the movie.