Just got back from seeing the movie. While it wasn't quite as good as I was hoping, it certainly wasn't the trainwreck that many reviews suggest.
It was funny. Not hilarious, but it make me laugh a few times and smile throughout most of it.
Things I Liked:
The dolphins' song: A funny little tune that reminded me of Monty Python, which is exactly what this movie should do.
The guide entries. They were right out of the book, and the little animations that go along with them are excellent. I only wish there were a few more.
The overall visuals: The movie has a great design aesthetic. I really liked The Heart of Gold, Marvin, Deep Thought, and of course the Margrathea factory floor was spectacular.
Mos Def as Ford: He didn't have a lot to do, but he makes an excellent Ford.
The new scene where they visit the Vogon homeworld: I thought it was very funny and fit right in with the established material. It worked well to add a second act to a story that didn't really have one.
Slartibartfast: Wow. The best performance in the film, very charming and funny.
The changed ending: I know a lot of people will bitch up and down about the changes made to the ending, but I honestly liked (most) of them, except for the little bit when that love story subplot comes creeping in.
Things I did not like:
The love story subplot: it's not necessary, it doesn't work, but it doesn't ruin the film either. It'll make you roll your eyes a little bit, I'm sure, but it's mostly harmless.
The John Malkovich part: Pretty pointless. But it's hard not to like John Malkovich.
Zaphod: Sam Rockwell was great, but they could have toned him down juuuust a bit.
General incoherence: They rush some stuff and don't really explain some other stuff, so the movie feels kind of disjointed.
So what we have here is a very likeable film that's on the edge of being really good but never quite crosses it. I know I'll go see it again though. If you were forcing me to grade it, I'd give it a B. High marks for effort, but it could have been just a little better. Nonetheless, I think Douglas Adams would have walked out of the theater satisfied.