I just got back from it. If you don't know the books, the movie will provide you with a few hearty chuckels, and little else. If you are a fanatic of the books, you'll love the movie...even though it isn't that good of a movie...
And now for clarification (because usually when I make posts like this, I get shit), I should say that just because a movie isn't good doesn't mean you can't like (or love) it, and just because a movie is good doesn't mean you will like it or love it. Within this logic, HHG is not a very good movie, but I did love it...