I really, really liked the movie. Don't expect it to be too true to the books, it's definitely the right style. Don't expect a lot from the plot (Adams is just silly, you know he is). Also, stay for the credits. If you haven't read the books, you'll miss a lot of jokes (although everyone I know that hadn't read the books but saw the movie anyway also liked it).
Also, Adams wrote the thing (co-wrote), which some people don't realize. If you're a big fan of the style, you'll like the movie. You probably have to be in a good mood to see the movie, though, because if you're not having a good day you won't be in the mood for silly.