Full disclosure: I have read all the Hitchhiker books, I've read the radio scripts, listened to the full radio series and watched the excellent BBC series.
The Hitchhikers Guide movie isn't bad. It's just not particularly good either. Working a book into a movie isn't the easiest of tasks, especially when you are dealing with comedy.
Cerebral wit is not easy to convey on film because of it's subtle nature. Douglas Adams was always at his best when he made you smile, he will not make you laugh out loud very often, but he wasn't really trying for that either. The Hitchiker movie crew made a valiant attempt at converting the cerebral humor to slapstick, making one wonder where Jim Carrey was.
The cast were all mis-cast. I can't think if one actor that completely worked, but they did try their best, and you can tell. Even Bill Nighy, generally very good in anything he does, seemed out of place. I couldn't help watching his interpretation of "Slartibardfast" as if it was a audition reel for the role of Doctor Who.
The script is really what lets the film down. There were many liberties taken with the original material, presumably in an effort to "update" it. But very little of the new material works.
What does work: the effects, while not completely mind blowing, are rather good. The opening number with the singing dolphins was quite fun, and made you wonder if this really was going to be better than you thought it would. The Vogons are very well realized, tho the performances of the Vogons themselves were variable. They were a tiny part of the book, but elevated as the main badies for the movie, perhaps that was the first serious mis-step.
I would recommend this film to someone who never read the book or watched the series. It may seem a lot more fun for someone with less experience with the original material. As long as they read the book and watch the TV series later.
5 out of 10 for the effort. If you were already a Hitchhiker fan, you might want to watch, but you will be disappointed, and a little sad for the missed opportunity.