A bit of a mixed bag, this one, and even its best moments are only moderately amusing.
Most of the problems are due to casting. Martin Freeman as Arthur has a tough challenge, playing an Ordinary Guy who's believably ordinary, yet somehow non-ordinary enough to hold our interest. Freeman's not quite up to it; his Arthur is solidly nondescript and far too bland.
Mos Def and Zooey Deschanel are pleasant enough, but Ford and Trillian should be British, and having them played by Americans, with American accents, was a very bad idea. Sam Rockwell falls completely flat as Zaphod Beeblebrox, playing him as a manic cross between Elvis and Bill Clinton.
There are a few good casting choices. Alan Rickman as the voice of Marvin and Stephen Fry as the voice of The Guide are both ideal (the animated Guide sequences are the best parts of the movie), and Bill Nighy is marvelous as Slartibartfast.
On the whole, though, a disappointment.