I finally got around to watching this movie the other night. As I watched, I thought it vaguely reminded me of a book by Douglas Adams which I'd once read. Lance Mannion does good work pointing out some of the major problems of the film, notably the transformation of Arthur Dent into your standard movie romantic hero schlub. Bleh. I strongly concur that Alan Rickman was great as Marvin ("I've been talking to the main computer...It hates me.") and that Zooey Deschanel should be allowed to sing in every movie she's in.
That said, taken as a movie unrelated to any book, it's still a mess. I'd see another movie featuring Sam Rockwell's character, though.