Review, short version:
It was a reasonably entertaining film, but was more 'inspired by' hitchhikers than a direct movie conversion of the book. Thus the reason that non-fhan's have been generally positive, while fhans have been scathing in dissaproval. Not a waste of money to see, but probably won't be on my collection list.
Review, long version (low spoilers)
Large parts of the story have been removed to make room for new parts which, in my opinion, are less entertaining than the parts they replaced. Vogons, which in the book only feature briefly right at the start, in the movie they triple up as the construction fleet, intergalactic police, and the butt of all the jokes, and the largest part of the re-write is a Vogon sub-plot.
Acting-wise, Martin Freeman who plays Arthur does a reasonable job for the most part, Mos Def as Ford is appalling and fails to capture the character in any way, turning him into a kind of black version of Tom Baker's Dr Who with a towel instead of a scarf, except not as good. Sam Rockwell as Zaphod is mostly piss-poor but has moments of brilliance where he nails the character perfectly. Zooey Deschanel as Trillian is wonderful, and for me portrays the character miles better than Sandra Dickenson did, but then the plot re-write turns her into a major character, a lot more fleshed out. Bill Nighy as Slartibartfast struggles in a role really not suited to him and tries very hard and nearly gets it, but not quite. Voice wise, Alan Rickman is Marvin is superb, Helen Mirran as Deep Thought falls way short of her usual standard and sounds bored and wooden. Stephen Fry as the book is adequate, as is Bill Bailey as the Whale.
Product placement for Nokia is pervasive and annoying to the extreme. In a movie set to a book based at a time where digital watches were high-tech, Arthur spends the first half hour of the movie in various poses with his brand-new, top of the line, nokia phone. Arthur wakes up in the hold of the Vogon ship, and the first thing he does is check to see if he has a signal. They get thrown out of the airlock, and the camera shows the phone, spinning gently through space (in a spin carefully calculated to show the word 'nokia' as much as possible). Nokia placement in The Matrix was perfectly done, subtle and effective, in H2G2 it was like being repeatedly smacked round the head with a nokia-brick.
The audience broke into applause twice. A large round of applause after a new pre-movie advert for Orange in the 'casting' vein they have been doing for a while now, which got the biggest laugh of the evening, and a small ripple at the end of the movie, which quickly died of embarrassment.
How much of the re-write was down to Adams I have no idea... some elements stink of his style of humour, such as the 'point-of-view gun' and the first few minutes of the Vogon sub-plot, but other elements stink of hollywoodisation.
Speaking of Hollywoodisation, they still seem to believe that people in England either live in cities, in castles or in farms. Arthur apparently falls into the last category, and lives in the middle of nowhere, surrounded by fields as far as the eye can see. Quite why, given the surrounding expanse of fields, it is necessary to build a bypass through the one house in a 3-mile radius is never explained, nor is how Arthur and Ford manage to walk down to a pub, drink 3 pints and walk back again in the 12 minutes Ford says they have before the destruction of Earth. Or even WHY they go to the pub, given that one of the changes they made is that Ford arrives at Arthurs house with a shopping trolley full of the required beer and peanuts. These examples of the changes they made to the existing story, in the first 5 minutes of the story, should give some indication of the extent of the re-write and the pointlessness of most of it.
The time 15:30/01/05/2005, The Place: Camden Odeon and the result: Not bad at all.
As I have often stated books or comics transfer badly to film. There are a few exceptions, which have been mentioned in previous posts (See Hellblazer/Constantine discussion).
I therefore tend to leave most of what I have read at the entrance to the cinema. Fact: The novel you read is your interpretation of the printed word. For those who harp on about the "Word", I suggest that joining a Christian Fundamentalist cult and spouting Dogma as the word might be in your future somewhere.
The result: I enjoyed the film a great deal. I was not offended by any artistic licence taken by the director. I do believe that some of the edits and extra bits made a work of Comedic genius filmable. I would happily pay to see it again in the cinema, due to the fact that I probably missed a good chunk of in-jokes the first time.
Casting: was great, parts: acted well, effects: outstanding and the film: definitely British in every sense (even Zaphod is played as the stereotypical English view of Americans).
I must at this point, highlight the biggest in joke of the film: It was funded by a Mouse Empire. Think about it: Disney funds a film about Mice that run a planet to understand the meaning of it all.
In short check it out. It is worth a look. I must also state that Mr Adams does make an appearance of sorts. Just watch for it, you can't miss it. Also they have set up for the sequel already. Ford does mention that he knows a great restaurant towards the end of the film, so I would expect that the Rodent Empire is watching the ticket sales. If we watch it they will make more..