Along with Terry Pratchett and J.R.R. Tolkien, he is probably one of the most beloved authors of all time. Many of Adams' works enjoy a kind of celebrity status alongside Monty Python as classic humour material and an integral part of geek culture... for better or worse.
Specifically of interest to the PPC, there is little badfic of his works, but many fans, especially young fans, quote him constantly. Sometimes quotes from The Hitchhiker's Guide to the Galaxy find their way into badfic simply because the author likes the book rather than because they're appropriate for the story.
In the PPC, May 25 is celebrated in part because it is Towel Day. This is a day where fans carry around towels to demonstrate how much they love the books and/or that they are hoopy froods who know where their towels are.