I first read the series when I was in Years 12 and 13 (17-18 years old) and found them totally hilarious. I cried laughing.
There is something about Douglas Adams humour that is slightly off beat and quirky that I loved. His explanations and asides to the reader are wonderful. On the downside, I was given the boxed set about 2 months ago, and although I've been reading and enjoying them, they're not as funny as they once were. I think my sense of humour may have changed, or maybe Terry Pratchett does the same things better. There's a definite comparison between the two authors, and maybe the comparison even extends to things becoming formulaic as the series progresses.
Then again, perhaps I'm being unfair?
Imogen