I honestly can't really find any faults with this book. Perhaps it's because I've never read anything quite like it before. It's 70% fantasy, 25% slapstick and 5% historical fiction. The main focus is on the plot and in this regard, Hughart has done a bang up job weaving a bizarrely entertaining tale. The quirky characters add spice to the already zesty story and the plot elements (gods, eternal life, ghosts, treasure, labyrinthine castles, flying machines, invisible monsters) have a wonderful ancient Chinese spin on them.
My only regret is that I don't know more about ancient China to find out how much of the book is based on real customs and places.