Rory posted an interesting counterpoint to my review.
Rory has some good points - I agree that you can't expect a movie to be totally faithful to a book, and I have noticed that there is a distinct correlation between how well somebody liked the book (and, perhaps, how familiar they were with the book), and how disappointed they were with the movie.
Rory would argue that we are "armchair literary snobs", but I think there's more to it than that.
Certain books, for whatever reason, resonate with certain people. Adams has a peculiar way of twisting reality in a particular way, and much of that depends on his choice of words. It's funny in a very specific way.
Somebody running around with a juicer on his head or holding the arm of a robot may also be funny, but not in the same way.
I'm disappointed with the movie because it didn't capture that particular view of reality (or unreality) that Adams excelled at.
To risk a poor analogy, I went hoping for a Kevin Smith movie or a Jim Carrey movie, and ended up with a Will Ferell movie. It's still funny, but definitely not in the way that I had hoped it would be funny. It's no longer quirky - it's mainstream, as evidenced by the major role that that Arthur-Trillian "romance" played in the movie, and it's total absence in the book. Just another "me too" movie...
Come to think of it, the book to movie analogy isn't really fair in this case. Hitchhiker's first showed up as a radio show, morphed into a book, a BBC TV series (which, for all of its faults, is funnier than the movie).