While it is true that buying a small tin containing casette tapes worth a hundred pounds may seem ludicrous, it is a treat worth investing in. Flog some shares, sell your cat (or owl) or remortgage your house - do what you can to buy this wonderful collection.
Stephen Fry has a wonderful, colourful voice whose intonation is perfect for bringing out the snide side of Snape, the dependable Dumbledore and the hard-headedness of Harry. It makes for laughs and excitement on two levels: the story itself and the dialogue on the one hand; and Fry's vocal gymnastics on the other.
So, suck a sherbert lemon, drive your Ford Anglia and listen to the Potter saga from a new and highly stimulating dimension.