This is one of my all-time favourite Pratchett books. Everything that he's good at is exhibited here - adapting stories and fairy tales, "translating" real life culture to the Discworld, great characterisation and dialogue, a great plot - and, of course, it's hilariously funny.
If you're a fan of the Witches, it's even better. Granny Weatherwax, Nanny Ogg and Magrat Garlick each get a chance to shine, in their own particular ways (as does Greebo). Pratchett takes them far away from Lancre, and it's highly entertaining to see how they react to 'forn' cultures and people. Nanny's postcards home are particularly worth a mention for sheer comedy value!
It's amazing how many references to real life cultures, stories and events Pratchett manages to fit into this book, and yet none of them seem hurried, forced or superfluous - each event fits in perfectly to one fantastic story. It's one of those books where every single line is a classic and every bit of dialogue is completely perfect. Hard to imagine how it could get better than this.