This is getting mixed reviews from the SF crowd, but I liked it a lot. Some of the people slamming it are big fans of the books, and they aren't happy that the film isn't 100% true to the books. Well, that's movies for you, and Douglas Adams did work on the screenplay, so it's probably as good as you could have expected. I read the Hitchhiker's books many years ago, and don't recall all the details so perfectly, so it worked for me. My 14-year-old daughter is a recent convert, has read the books multiple times in the past year or so and heard the BBC radio series, and she loved it, even though she's very aware of what was changed.

If you can laugh at SF and the Monty Python sort of humor, then go and see it.