I'm a huge Douglas Adams/Hitchhiker's Guide fan (note my avatar), have been since stumbling across the T.V. version back in the early 80s when it aired on PBS. That made me read the books. Later, in college, I got a chance to listen to the original incarnation, the BBC radio program (roommate had it on tape)...that's the best version by far, IMHO. Impeccable writing, dead-on comic timing and nice audio/effects production values.
The film. Nicely done. Big problems--1) it felt 'rushed' story-wise, and understandably so. In all the other versions, it was episodic and moments/scenarios could be set up to great effect, and 2) because of this, it comes off a bit muddled, with an 'everything but the kitchen sink' feel, and 3) it nonetheless ends quite abruptly and all the story/character arcs are woefully incomplete. This makes me wish that it does well enough in video release that they are able to continue the tale...it needs it. In short, I liked the film, but can see why folks unfamiliar with the Hitchhiker story would come out confused. This could be remedied with further elaboration via sequels to fill in the character and story questions raised in this, by design, introductory chapter of a comedy/sci-fi epic.