I suppose it's a tricky business, adapting a book in general, and adapting one for nerds in particular. On the one hand, and especially here, the people who will most likely see it will be the ones who've memorized the book, and will be expecting the movie to be exactly the same in all respects. On the other, if a movie is exactly like a book, why go see the movie at all?
I digress because the movie is not entirely faithful to the novel. There's a subplot in there about the characters going to Viltvodle VI and the candidate Zaphod defeated in the race for the Galactic Presidency (which is actually one of the two big episodes of canon-rape in the movie...but then again, I always thought the bits about the Galactic Government in the novels were some deliciously vicious political satire, and so it makes me a bit sad that they got monkeyed with here. Though they do have Ford saying "The job of the president is not to wield power, but to draw attention away from it", which I'm glad they left in.), leading to Trillian being kidnapped by the Vogons, and.... yeah. These scenes do work on their own, but I was left with a vague feeling of "...wait a minute, that's not right" about it, at least at first.
The major instance of canon-rape is... well, let's be blunt--the contrived romance between Arthur and Trillian. It doesn't exist in the novel, the radio show, the computer game, whatever. And there's no reason that I can see that it should be in the movie. ...unless Douglas Adams thought it should be there at the end, which would make things so much more complicated, and would make me feel guilty for not liking it. And at least it's not nearly as bad as it could have been, since most of it just takes the form of attraction-from-afar and unrequited longing.
All right. So much for the bad stuff. Now on to what I liked!
First off, the Guide scenes are some of the awesomest awesomeness that ever was awesome. I love how they did them. And I get the definite sense that whoever designed those scenes was having a whole mess of fun.
They got the look and mood of the movie right, I dare say. I like how things looked, especially the Vogons and Deep Thought. And the Magrathea scenes.
The Improbability Drive scenes were pretty damn funny. As was the scene with the whale.
Alan Rickman, as Marvin's voice, does an awesome job.
I like the beginning, with the dolphins singing "So Long And Thanks For All The Fish," which is a rather catchy tune.
The main characters, on the whole, stay in character throughout the movie, which I appreciated. And Martin Freeman is just excellent at Arthur.
So, to sum up: Don't go in expecting the movie to be just like the books, but it's entertaining and worth the money.