Having seen the movie before I read the book, I did find the movie to be reasonably entertaining and often funny, but just the sort of light smile funny and not laugh-out-loud funny. I actually kind of liked the film's interpretation of Zaphod's heads, since it caused the second head to be more of a minor running gag (as it was in the text- and audio-based versions) and less of a constant distraction. I was never a fan of the largely unexplained presence or absence of his third arm though.
Attempts to rewatch it after reading the books have usually ended with me getting bored and turning it off during the plodding middle act, so I do agree that the joke density could stand to be a lot better.
"I think it's the idea that counts, not the actual method. As long as you set out to cause a horrible death through insects eating the person, you're good." - apenpaap