According to the bit in the back of the book, this novel has become a "cult classic" since its publication. The book's sales rank on Amazon supports this claim. I haven't read any of Pratchett's books, but I've read a few by Gaiman. I didn't like this one as well as those.
Gaiman has a talent for taking mythological themes and making them believable. But this book is more of a farce, and as such, it failed to suspend my disbelief. This story, like others by Gaiman, draws from mythology, but unlike American Gods, the mythology isn't Norse, African, or Hindu, but Christian, and that will offend some, or at least cause discomfort. Believers don't like to see their beliefs treated like myths.
Good Omens is about a friendship between a demon and an angel. Neither sees any sense in a war between Heaven and Hell and prefer to thwart, rather than assist during Armageddon. Crowley, the demon, and Aziraphale, the angel, have lived amidst humanity for so long that they no longer see things in such black and white terms as pure good or evil. They no longer fit in with the bureaucrats of Heaven and Hell. Unfortunately their sophisticated viewpoint isn't universal; satirizing conventional behavior just doesn't work for me.
I find no humor in society's increasing polarization of beliefs and attitudes, in the absence of dialog between left and right, religious and secular, rich and poor. Envisioning Heaven and Hell populated by rigid thinking, bureaucratic zealots simply doesn't amuse me. The world is full of such people already and their numbers are steadily increasing. Maybe Armageddon is coming after all, and that's just not funny.