I'll admit, Garth Jennings' Hitchhiker's Guide to the Galaxy was much more entertaining than I expected.
Martin Freeman's Arthur Dent is actually a more likable protagonist than his literary counterpart. Mos Def plays a charismatic but bland version of Ford Prefect. In the novel Ford is a jaded, slightly psychopathic hedonist. Whereas, in the movie, Ford is primarily Arthur's cool buddy. Sam Rockwell's Zaphod Beeblebrox is an embarrassing cross between Jeff Lebowski and Maj. T.J. Kong* from Dr. Strangelove. Douglas Adams wrote Trillian as a brilliant mathematician/astrophysicist who was bored and kinky enough to run off with a two-headed alien. Basically, Zooey Deschanel's Trillian is pretty. The romantic subplot is so bad that I prefer to read it as posthumous "fuck you" from Douglas Adams. (I need a romantic subplot? Right, then, we'll give it three scenes.)
Bill Nighy gives HHG's best performance as Slartibartfast. His Magrathean planet-construction sequence is the highlight of the film.
The art direction and set design are hit and miss. The good guys' technology has a satisfyingly bulbous look. The design is timelessly nifty--retro, yet futuristic. (That's no mean feat. To look futuristic in 2005, you can't just recycle what people envisioned the future to be like in the late 1970's.) The marshmallowy Heart of Gold spaceship, the fire engine red explorer pod, and Marvin the depressive robot resemble Murakami's Superflat sculptures.
At this writing, everyone is raving about those Vogons. Agreed that Jim Henson's Creature Shop executed the Vogons beautifully, but I wasn't quite as impressed with them as Wyld Card, et al. Too much evil, not enough banality of evil, if you ask me. I hate to say it, but the Vogons' environment looks way too much like sets from Fraggle Rock. No disrespect to Jim's FR, but the Vogons' digs are just too squalid for a race of faceless corporate bureaucrats. (And call me a fangirl, but I had my heart set on a yellow Vogon constructor fleet.)
I applaud Jennings' decision to use animated "entries" from the Hitchhikers Guide to explain some of the more obscure plot points, including the Infinite Improbability Drive. The HHG novels are less emphatic on this point, but the central conceit of the radio play is that the listener is hearing the story from the Guide itself.
Happily, the same actor supplies the voice of the Guide in the movie as in the original radio drama. (Turns out I was mistaken about the identity of the Guide. Thanks, Johnny.)
Unfortunately, most of the actual animation sequences look pretty stupid. The animators were obviously going for a retro vibe, but the end product was an artless grab bag of the worst graphic design cliches from the 1950's to the present.
It's hard for me to judge the movie except as an adaptation of the book, but I get the feeling that it wouldn't be very satisfying to anyone who wasn't already a fan. In the novel, the plot serves primarily as a structure for witty digressions, riffs on philosophy and physics, a huge cast of minor characters, and wordplay. The movie omits most of that, not to mention the clever non-linear storytelling devices. It also adds a lot of slapstick.
HGG was obviously not an easy story to adapt. Making a movie about Adams' universe is like trying to project a globe onto a two-dimensional map. Considerable distortion is inevitable. The movie is remarkably successful at suggesting the eccentric charm of the source material without actually showing very much of it.
*Historical note: Slim Pickins' beloved nuclear cowboy is also thought to have been a major influence on G.W. Bush's "43rd President of the United States" character in Dick Cheney's America.
Update: David Edelstein has an interesting review in Slate.