If I were to say that, even now, nearly 15 years after I first read it, this book is still one of my favourite reads ever, you will probably get some idea of the direction this review will go. Just imagine, if William Friedkin had made a film of the Just William books - that's what this book is like.
The mix of Pratchett and Gaiman is pretty much flawless, with all the sparky wordplay and fun of the former mixed with the mordant, grim wit of the latter. Put together they spark, like Crowley and Aziriphale, even though they really shouldn't.
The highlights are too numerous and fine to count, but it's a good sign when there's a laugh on almost every page and even the footnotes are a riot; the beginning of the book is a prime example, the Earth's a Libra indeed...
I think this is probably one of those books that everyone should read at some point or other and one that is filled with a great deal of love and a sense of fun about the genre and characters it parodies so relentlessly.