So, my first, and most important comment would be to just forget the movie and read the book instead.
I don't think the movie makers did a bad job with it--they definitely tried hard to keep the movie in the right spirit as the book, but I think the movie was sort of doomed to...not succeed (I don't want to say fail b/c that's just too strong).
Basically, as one of my friends pointed out earlier today, Hitchhiker's Guide doesn't really have a lot of plot. What holds the book together is sheer randomness/hilarity. A lot of the details that made the book work just plain had to be cut out for the movie.
So basically, the movie ended up just being a set of random scenes--without the "glue" that held everything together in the book.
All that being said, Alan Rickman is just fantastic as Marvin, and I'm noot sure the actor's name, but the person who played Ford was fantastic :)
All that being said--it honestly won't take much longer to read the book than watch the movie (the first book in the series--which is what the movie covers--is only about 120 pages, but it's really really fast read), and the book is just so much better!