GO SEE IT NOW.
Or, perhaps you're looking for something a little more in-depth? Fair enough, I'll even leave out the lovely spoilery details.
The cast is excellent. I was quite worried about the choices for Ford and Trillian, but they worked wonderfully. Mos Def is on par with the original Ford from the TV series because he doesn't even try and compete, he takes the role and just goes along with it. He acts well, he makes a rather good alien and he just fits. Same goes for Trillian.
That's the wonderful thing about this movie, it doesn't try and compete with the radio series or the TV shows. It's just another segment of the whole. If you're book loyal and want to go in to see something entirely faithful to the written version you're going to be sorely dissapointed. This is NOT the book. This is what Hitchhikers probably would have been if it has always been a stand alone movie. There are bits missing, there are bits added. Clearly you couldn't fit an entire 'trilogy' into one movie and it picks up the pace in some areas. Things are changed around to make it more....hmmm....movie-like. But it all works.
Also, I love Marvin.
There are a few notable things to watch out for. I love the intro with the dolphins. There is a rather fantastic cameo in the queue scene (you'll recognise it when you get there) that had me in a fit of squees. Stick around for the credits also, there's a little bit extra from the Guide itself there. Lovers of the original will recognise the theme music when it pops up. Douglas Adams himself makes an appearance at the very end too, which I think is a wonderful tribute to the man.
It could stand alone as a movie easily, which those who have not read the books (and shame on them!) will still enjoy. Naturally I do miss some parts of the original story, and some of the changes had me a little confused at first but it's very easy to see why they removed parts or added things. I love it and will most likely be preordering the DVD as soon as it is available.
Incidentally, I have a feeling slashers are going to have even more of a field day with Ford and Arthur than they already did.