Saw Hitchhikers last night. Now, I've never read the book, nor have I seen the TV show, so I can't really judge how fans would feel... but I loved it. I hear the book has a lot of Monty Python-esque humor, and it's clearly evident in the movie, especially the opening number (there's only one musical number, in case that gives you any reservations). I loved the snarky yet stuck up narrator, who gave lines like "In the beginning, God created the universe. It was highly considered a big mistake" (or to that effect) and my personal favorite, "In the event that you find yourself hopelessly in need of rescue, remember how good life has been to you so far. Conversely, if it hasn't been so good (more likely given the situation), don't worry. It won't trouble you any more."
Muppet fans will be pleased to hear that the Jim Henson Creature shop provided the creatures, mostly the Vorgons (did I get that right?)
I'm leaning to a theory that it's based more on the TV show than the book, because the Marvin suit (as disclosed in the credits, "original Marvin robot costume provided by BBC (?)") is taken from the series.
Over all, I really enjoyed it. The worst I can think of is that it's a good popcorn flick.