Good Omens is a fun guilty pleasure book. It's a real "you got your chocolate in my peanut butter" when you mix Pratchett & Gaiman together. They are both fun to read authors in their own right, and mixing them together does provide fruitful results. You can often tell which author wrote which parts but that isn't really a complaint. It's a fun pairing and I wish there were more author mashups out there to find. (Maybe there are, I intend to look into it).
The book is light and breezy despite the potentially heavy subject matter (the end of the world, angels and devils, the anti Christ, etc...) and it makes for a quick light read. The jokes don't land as often as I would have liked to but there are more funny parts then there are misses. The weakest stuff in the book is anything involving Adam Young. I just couldn't get behind the eleven year old boy being the one who has to bring about the end of the world. Every other character was fun to read. Particularly Crowley and Aziraphale. It was lots of fun to read the back and forth bickering of an angel and a devil with a love hate relationship.
If you like either one of the authors, I think that you'll like this book.