It was fun, it had SF authenticity (the way Galaxy Quest had) and the performances were good. It had a plot, it was funny, the characters were well conveyed. There were a small number of daring devices, used to convey the important but slippery concept of the improbability drive, which I think is the key to the sequence in a way. There were three or four points where the 'space: it's big' idea is well conveyed. I love that in film.
Sure there are bad moments - I hated the Hollywoodisation of the Arthur/Trillian relationship. Spew. A few jokes are fluffed. I wasn't utterly convinced by Marvin and his plot epiphany.
I was worried that Martin Freeman was a one-Tim wonder, but he Arthured up very fetchingly, giving a different interpretation to the character than Simon Jones. His Arthur Dent is less passive and useless, though (even) more cowardly. I thought Trillian was fairly well written but Zooey Deschanel's performance was bland.
I thought Mos Def was fine as Ford Prefect. He was alien but a guide. There is incidentally no question but that he is supposed to be attracted to Arthur. However the on-screen relationship may be cool but it is not hot. It's just a thing. Well, they don't have to design everything to appeal to my lower nature do they?
I've saved my best to last. I adore the character of Zaphod Beeblebrox. Remember that question about what character in fiction are you in love with? I've always adored Zaphod. The thing about Zaphod is: how much of that superficial charm is a bluff, and how much is it a double bluff? Is he pretending to be stupid, to cover up his ability or his inability? He doesn't even know himself. Sam Rockwell, whom I know nothing about, is rumoured to have not 'got it' during filming. I can't say, but I thought this complexity was conveyed, perhaps accidentally, by the performance. I know other commentators have disagreed.