There are so many things about this book that make it a classic. Besides the humour - which there is plenty of - the social commentary is extraordinarily insightful. As with the case with a lot of science fiction, the universe provides a new venue for us to examine ourselves. Adams did the same thing as most sci-fi writers - he just did it with a hell of a lot more wit.
Bottom Line? HHGG is absolutely hysterical. And like the best humour, it has a very truthful ring to it. Also, reading it will let you in on all sorts of jokes that you have been missing all these years!