The film has its flaws, more of which in a moment, but it has its good points too.
First, the set and costume design, the effects, the cinematography, and just in general the whole look of the film. Near perfect, IMO. It recalls the TV series in places, but has a style of its own too. The Vogons and their ships are terrific - ugly and brutish. The Guide is perfect too - not too hi-tech, with many nods to the TV version. Everything in this department works. This, I reckon, Douglas Adams would have loved.
Second, the casting. Sam Rockwell as Zaphod grew on me after an initial doubt. He has an insane but undeniably charming grin, which helps. The major problem is that he lacks chemistry with Martin Freeman. He still exhibits genial contempt for Arthur, and there's obviously supposed to be friction between them, but it doesn't come off very well. Still, he fills the role well.
Martin Freeman does his usual schtick. He is onto a loser from the start, in a sense, because no-one will ever match Simon Jones' Arthur: a masterful depiction of bemused but stiff Englishness, a man who looks vaguely uncomfortable in his own skin. Freeman only gets close with one line - "I'm English, I know how to queue!" (It doesn't help that Jones gets a cameo, reminding us of the definitive Arthur.) But he does OK.
Zooey Deschanel works fairly well. She's pretty without being stunning, which is what Trillian should be. It's important to note, as I realised early on, that Trillian in the film is basically actually a composite of Trillian and Fenchurch. Once you accept this, the role works better.
Mos Def as Ford is a disaster.
The secondary casting, however, is perfect. Alan Rickman as Marvin nearly steals the film, Bill Nighy as Slartibartfast is terrific, Stephen Fry as the Guide is an obvious but hugely successful piece of casting, even Bill Bailey as the whale is incredible. If only they hadn't cast Mos Def as Ford...
Now, the plot. It's important to realise that this is a different version. One of the problems, in fact, is that the first fifteen minutes or so sticks too closely to the book (apart from a strange musical number with dolphins), and appears to be an abridged summary with most of the best lines cut (though there are still a few classics - "none at all", "I don't know, is it the sort of thing you're likely to say?", "aren't we supposed to lie down with a paper bag over our heads?").
Until we get on board the Heart of Gold, then, you get the impression you're watching an inferior version of the TV series. But the plot then gets into its stride, and goes its own way. New characters, new arcs, new lines even. And I actually quite liked the new story, for what it is.
The script lacks the clever, sharp wit of the original, and doesn't revel in surreal conceptual digressions in the way we would expect from DNA. (OTOH, in the latter case that wouldn't have worked well in a film anyway.) But it is just much less... whimsical than the other versions, which is a shame.
There are one or two nice nods to the TV series - the original Marvin appears in a queue, and Simon Jones plays the recorded announcement from Magrathea.
Overall, I'd say the film was well worth seeing, and Mos Def aside (did I mention he's a disaster as Ford?), is as good as could have been expected.
