I've read many thousands of SF/F books. I put this at the apex. For context on my opinion, the quality and depth of the tale is above even:
#2. The Hitchhikers Guide to the Galaxy (Douglas Adams),
#3. The Princess Bride (William Goldman),
#4. Job: A Comedy of Justice (Robert Heinlein),
#5. Bloodsucking Fiends (Christopher Moore).

Plot Summary: There are many good descriptions of the plot on Epinions, so I'm going to focus on the meaning of the novel, rather than the individual events and characters in the story itself.

Premise: The books of Revelations and Genesis are 100% accurate, though a few key passages were lost in editing. God, Lucifer and the whole heavenly host are all very real, and Armageddon is a few days away.

Authors: When I describe this book I usually say, "Pratchett wrote Gaiman's vision." Gaiman's works explore his belief that reality is a story we tell each other. Pratchett's are filled with wonder and absurdity - particularly his wonder that the absurd often feels more real that reality. This collaboration represents a transition point in the writing of both men; if they were singers, I'd say their duet gave them both an extra octave of range. Good Omens makes very serious topics very seriously funny.

Writing: The tale itself is both predictable and surprising (much like life). Predictable: every one of the large cast of characters is developed with profound and hilarious clarity, so much so that every decision and interaction seems inevitable. Surprising: the characters are characters, with failings and strengths, motives and emotions, desires and delusions; the individual trajectories of their lives combine into something much greater than any one of them.

Philosophy: Good Omens is a thought experiment which tries to answer the Question - the one that's hard to write down without circling: "What Makes Us People?" "What Is Reality?" "What's it all about, anyway?" (Thanks to Douglas Adams.)

Regardless of the ambiguity of the Ultimate Question and the hilarity of the tale, the parameters of their experiment are clear and taken very seriously. (One power of fantastical literature is this ability to play with unreality in order to explore a facet of the human condition - Star Trek is a popular example.)

The first parameter is mentioned in the premise, above. If Genesis and Revelations are real, angels (fallen and not), miracles, prophecy, wars in Heaven and Hell, Creationism and Armageddon are the real story. It means that God has a Plan, and it includes testing humanity to destruction.

The second parameter of the thought experiment is that everyone believes "I am normal," and deals with whatever reality they encounter from that belief. We incorporate the discontinuities into the story we tell ourselves, and call that 'life'.

Conclusion: In the end Good Omens says "Reality is what we say it is," but the blow is softened by exceptional writing and good humour.

Recommended: Yes