This is less a comprehensive book review, and more a book opinion. My tastes will differ greatly from yours, and may even change over time.
I wish I liked this book more. It was really good, but I didn’t get the same level of excitement other fantasy fans have for it.
I have a taste for the silly and for nonsense. I love asides and footnotes and parentheticals, fleshing out stories and knitting them together in grand ways. The bubbling prose of Adams and Pratchett does it for me, and I do like Anansi Boys (one of my faves) and the inventiveness of American Gods.
Again the general plot of Good Omens sounded great, and the book was alright, with some odd jokes that made my 2019 brain grimace but were nearly all fixed in the TV show. In places the book was amazingly witty and I did laugh out loud on many occasions. It was a really good and vivid book. I wish I could tell you what I was missing.
I read the book, and then watched the TV show. It’s an odd thing. That whimsy didn’t quite work for me in the book didn’t on the screen, either. I think it was the love of Crowley and Aziraphale and all their depths, contrasted with the very silly-but-great-in-their-own-right other characters that didn’t help. In the book it’s wasn’t quite as obvious, but the screen really teased it out,
The climax of the book was very abrupt. I went back to reread and check, and even though that kind of mischief is what I expect from these two authors, I don’t think it landed for me.
Overall I’d recommend this book, but if your tastes perfectly match mine, go in with lower expectations than the premise gives.