I've only read about seven of the Discworld books, so I'm hardly an expert, but I know enough to feel qualified to write this review. I feel that this is better than any of the other books of his I have read. While his books are all wildly original, this one wins on the points of comparison.
First off, the settings of Ankh-Morpork and the Counterweight Continent are both well done. Pratchett taps into the same vein of humor underrunning the image of Imperial China that Barry Hughart did in "Bridge of Birds". He also introduces a very interesting linguistic idea, playing off the use of tone in spoken Chinese to create a language with few words but many, many different pronunciations. The other plot ideas, such as the Mandelbrot Butterfly etc., are well done, but the language was such a well-thought yet off-the-wall idea that it beat everything else.
The characters are well done also. Old barbarians are a comic image, but Pratchett is able to really use his old barbarians to advance the plot rather than having them hang around solely for laughs. The Gods play off each other well, and Rincewind's cowardice and fleeing make the places he lands in all the more interesting. The ending wraps everything up well without being too perfect, and Death is funnier than ever.
Hitting nary a wrong note, this book is worth getting out of the library, or buying if you are a Discworld fan. I would recommend it to anyone who likes fantasy, and also "Thief of Time", by the same author.