I first heard The Hitchhiker's Guide to the Galaxy on the radio in London. I went on to the tapes, the books, and even saw the TV show and movie. The non-video versions, with all their differences, were terrifically funny.
Here, Adams branched out into something not Hitchhiker-related. He kept the strange and paradoxical, and attempted similar humor. Unfortunately, it just didn't work as well. Where Hitchhiker had the infamous Infinite Improbability Drive to explain things, this book doesn't, and it shows - strange things just happen, but for no really good reason. The result is an amalgam of gimmicks and gags that is only mildly amusing.
Some parts of the book are fun - there are extended references to Coleridge, and some nice parody of English colleges. A lot of the right elements are there. But it doesn't all hang together very well; the connective plot is thin, and in some cases possibly absent.
All in all, a mildly fun book for Adams fans, but skippable for casual readers.