I received this audio book as a gift from a coworker; she had acquired it for (she says) a quarter at a flea market. Whatever the monetary value, however, this book is a little piece of history to me. It is perhaps the most tangible connection I will ever have to the BBC radio series of h2g2.
The Hitchhiker's Guide to the Galaxy as narrated by Stephen Moore is a wholly different experience than any other edition of Hitchhiker's. Of course, this is true of every edition of h2g2--it changed with each medium (anyone who has read the books and listened to the radio series, or seen the movie and read the books, knows what I'm talking about). The difference in this case is that the story did not change, but suddenly it has a context: it's narrated by an Englishman.
Stephen Moore has a history with h2g2; he was not just some random English actor chosen for the role. And in fact, he is perfect as the narrator. Hitchhiker's takes on a life of its own under his narration.
I suppose I should at this point note that I enjoy audio books. Not everyone does. And it's true that they are largely hit-or-miss--the book requires a good narrator, or else it ruins your experience of the story. However, audio books are both an enjoyable and a practical experience. You can listen to audio books when ordinary reading is out of the question (trust me: reading and driving do not mix). So when I extol the virtues of this audio book, it is from the standpoint of someone who will actually listen to audio books. If you dislike audio books (or if you don't own a cassette player...), I'm not sure this one is good enough to make you change your mind.
If you like h2g2, however, I would definitely recommend listening to this if you ever get a chance. Even if you've heard the radio series, this is slightly different, because it is the book. It's nice if you've a quiet night at home with nothing much else to do. Make a cup of tea, pop this in the tape player (yes, those still exist), and press play.