I just got back from the movie. It was good, but I think I liked the BBC TV show better. It wans't a glitzy and the fx were cheezy, but it stayed truer to the humor and plot of the book. But, if you don't have a preconcieved notion of what "HH2G" should be like I think you will like it a lot.
My advice: watch the movie, read the book, then watch the 80's TV version.
I agree with the above post.
The only thing I would change is the order. I would watch the movie, watch the 80's TV version, THEN read the book.