Like a few others, I went and saw the Hitchhiker's Guide to the Galaxy this weekend. I took my parents (who were in town for a brief visit) and none of us had read the book or knew anything about it (except it's a cult classic). I've read a lot of criticisms about the movie today on blogs, but most seem to be related to the book vs. movie thing. IMO, the movie is rarely - if ever - going to be as good as the book.
We all enjoyed the movie and I laughed my ass off throughout. I thought the characters were great and the special effects were incredible. I'd see it again w/o reservations. However, I'm easy to please when it comes to movies - so you might want to take my opinion with a grain of salt.