The Hitch-hiker's Guide to the Galaxy is great. That is to say, the radio play, the novel, and the TV series are great. The film is mediocre.
Maybe this opinion is just a product of my own encyclopedic knowledge of Hitch-hiker's (when I was fourteen I read the first four books in one day), and I was simply judging the film from my own preconcieved notions of how the film 'should' be. Certainly no-one wanted this film to be good more than I. Although I actually wonder how anyone who hadn't read the books would follow this film at all- the friend I saw it with, who has a passing knowledge of the books, was often a little lost, and I can't say I blame her.
It's not out-and-out awful: there were plenty of moments that could have been cringe-inducing, but weren't. But it's not very good, either. I think the main problem is that it's very rushed. One of the wonderful things about the novel was how well paced Adams was. He was never afraid to take a little non-sequitur when he thought it might be diverting. Indeed, you might say that was the whole point. While the film does have the occasional wander through the pages of the book, the rest of the film feels like a mad rush from planet to planet for some mildly funny set-pieces in awfully contrived circumstances. You may say this rush was necessary to fit the entire book into one movie, and you may be right, but that doesn't exactly fix the problem.
Casting was good- Stephen Fry as the book is inspired and easily the best thing about the film overall. Sam Rockwell is the perfect Zaphod, although he spent most of the movie being less 'cool' and more 'insane'. Mos Def did a passable Ford but again, in comedy, timing is everything, and the timing just seemed...a little off. I mean, in the novel, Arthur and Ford are trapped in the Vogon airlock, and...
"So this is it," said Arthur, "we're going to die.""Yes," said Ford, "except ... no! Wait a minute!" he suddenly lunged across the chamber at something behind Arthur's line of vision. "What's this switch?" he cried.
"What? Where?" cried Arthur twisting round.
"No, I was only fooling," said Ford, "we are going to die after all."
...see, that's funny.
In the movie, there really is a switch. Ford doesn't say "just kidding", he actually investigates the switch and discovers it doesn't do anything. That's not funny. And it shows a worrying misunderstanding of Adams' humour on the part of the film's creators that occurs repeatedly throughout the film.
It's amiable and has some good effects and some nice moments. It's just not that funny. (Loved those sighing doors, though)
This is a 
Fair review. I thought the movie was entertaining but like you, rushed.
I thought Zaphod was terrible and Rockwell annoyed me in every scene he was in.
Still there wase some good comedy in it, even if a lot was missed.
And Trillian was a babe.
I must admit I really liked Trillian and thought she was an improvement on previous incarnations. I also liked the whole 'feisty adventurous chick helps stodgy British bloke to see that a life of exploration and challenge is more exciting than one of staying at home with your cup o' tea' subplot a lot more than people's complaints had led me to believe I would.
Although almost every reviewer hated Zaphod, I thought he was excellently portrayed. Ian and I saw this and both loved it.
The best part of the whole movie was at the end, when the one mouse goes:
"Take his brain!"
:)