This is a satire of the Christian Apocalypse. It's certainly fertile ground for humor, and there are some good jokes in it. And there are even a couple of meaningful points about the nature of humanity. The story moves fairly well, and it feels a bit like Discworld.
The lame, boring parts of a Discworld book, between the memorable, funny bits, that is. Because, for all the potential in this book, it really doesn't work for me. I think that it being set on present-day Earth makes the goofy stuff too hard to accept, and while I've absorbed enough Christian lore to get at least most of the jokes, I never invested so much time in studying that stuff for parodying it to have the full effect on me. The good jokes didn't make me bust a gut, they just made me smile. And the general run of the story bored me. 5 out of 10.
**** PLOT SUMMARY -- SPOILERS ****
Crowley is a demon and Azariphale is an angel, but they've both lived in the mortal world since Creation in 4004 BC, and they've gotten to be fonder of the world and of each other than of their sides. They're hapless and lazy, but for some reason their respective bureaucracies trust them. So when the Antichrist is brought into the world as a newborn, they are given the task of placing it with the human parents where it's supposed to be raised with the proper bad influences. But they screw it up, swapping it for the wrong baby, so the important American diplomat who's supposed to be raising the Beast keeps his normal son, and an unsuspecting middle class British couple get the devil's child. There's a kind of funny bit where the Hellhound arrives, and Adam (the Antichrist) makes it shrink down into a terrier mutt to fit his expectations. The Four Horsemen show up (except that War is female) and have their Attributes delivered by FedEx. The baby who was supposed to be the Beast is brought to the Middle East and nothing happens. Crowley is visited by Beelzebub and Hastur and manages to evade them. These and a bunch of other characters converge on Lower Tadfield where Adam lives, and Adam finally realizes what's supposed to happen and decides that he doesn't want it to happen. And somehow manages to have enough power to convince the hosts of Heaven and Hell to just go away.
There were a bunch of other characters that we followed, but I don't really feel like writing anything about them.