Film The film: Succeeds at everything other versions failed at and fails where they succeeded.
Let's start with the good, if only because there's so little of it: the special effects are beautiful, the set designs awesome, and the puppetry amazing. It's nice to see Trillian get rounded out even slightly more than she was in the past and Zooey Deschanel is great in the part. The music was great, most specifically Joey Tabolt's cover of the iconic Journey of the Sorcerer and the opening number (the lyrics of which sound like they came from Adams' own pen despite having been written by one of the directors). The scene on the Magrathean factory floor is absolutely breathtaking and Bill Nighy and Bill Bailey nail their parts.
Moving on. My primary problem with the film is the writing. It may have been written by Adams, but it doesn't feel like Adams—most of the wit and satire has been stripped out, and most of the classic gags have been shortened beyond recognition. Most of the new jokes are just kind of stupid or predictable. This wouldn't be a huge problem, except that Hitchhikers is and always has been all about the jokes. Any plot there is just strings the jokes together. The film, however, plays the plot as priority, and suffers for it. Most of the bits that are added to solidify the plot are dull and, in fact, the plot picks up and forgets more than enough threads to make one wonder why they were necessary to begin with. Because the pacing is tighter to accommodate this plot, the Adamsian bits that got left in—the whale scene, for instance, or the Guide entries—feel out of place and kill the pacing that everything else got murdered for the sake of.
There's also the general feel of the film, which is generally more optimistic than H 2 G 2 ever was. Space and technology are treated as wondrous and exciting, as exemplified by Trillian showing off the Heart of Gold's gizmos and Arthur making the uncharacteristic decision not to stay on Earth once it's rebuilt. The franchise up until this point played space society as just as bureaucratic and corrupt as it is on Earth, with any upbeat moments being the result of love, principles, and philanthropy rather than everything generally being hunky-dory. This is akin to making the Star Trek-verse dystopian or the Firefly-verse Lighter And Softer.
I have many more issues with this film, but those are the biggies and I'm running out of words.
Film Film: A very American adaptation of a very British story
After having viewed the movie several times, as well as listened to the radio dramas (the original ones as well as the ones adapting the three later books), and skimmed through the books themselves, I think that the main problem with the movie can be summed up thusly:
It's a very American adaptation of a very British story.
The original Hitchhiker's story was a very black, ultimately pessimistic but extremely sharp and often subtle "people are a problem" satire by the very British Douglas Adams. In the movie the satire, while still present, is underminded and often contradicted by a much more American/Hollywood attitude — which is broad, blatant and much more optimistic: Here you have a world where only the villains are truly bad, the good guys may be flawed but come around and change for the better in the end, and in true Hollywood fashion, the "dorky average guy" that the audience is meant to identify with saves the day, becomes braver and cooler, and of course gets the hot girl.
And so we have two very different philosophies struggling for attention here, and this makes for a very uneven tone and feel all around. Are we supposed to root for these people, or shake our heads at their folly, or both, or neither? When Trillian gets all hurt and disappointed that Arthur doesn't want to quit his job and go to Madagascar with a girl he's known less than twenty minutes, are we meant to side with him or with her? The moviemakers don't seem to know either.
That's not to say the movie is without its merits. It looks gorgeous; the Vogons in particular look awesome, and the scenery is often breathtaking. Most of the actors do good jobs, particularly Martin Freeman, Alan Rickman and Bill Nighy. There are also several good ideas here, like the POV gun and Humma Kavula — and the Guide segments are brilliant; I know people have chastised the movie for dumbing down the orignal material, but when I first saw it I was pleasantly surprised how much of the original wit and humor had actually been preserved here, especially when the Guide was involved.
In the end, I'd say the movie's worth at least a watch for its strong points, long as you accept that it is a Hollywood movie trying to adapt a very non-Hollywood story... with all that entails.