I did enjoy this, but sometimes I get the feeling I like Terry Pratchett and Neil Gaiman themselves more than I like what they write.
The humor was amusing (though never hilarious), the set-up was interesting, I liked (most of) the characters and the plot was intriguing enough to have me following it once I got involved, but it never quite "clicked" with me and I never managed to love it.
Crowley and Aziraphale's rather interesting relationship was, in practice, the center point for the novel, and I did enjoy it. They had the feel of old investigator partners, where they maybe don't really like each other, but they are used to each other and to one playing "good cop" and one playing "bad cop." It is a comfortable sort of relationship, which is why they aren't too pleased with the idea of the Apocalypse interfering with the way things are going for them.
The Anti-Christ's story line was also kind of interesting, although he himself kind of...wasn't. I'm not sure how you can manage to make the Anti-Christ an uninteresting child, but there you have it. I was faintly annoyed every time he came on screen, not because I disliked him (I feel very weird saying that I didn't dislike the Anti-Christ, but somehow it is that kind of book), but because his character just never did much. I think I was supposed to find him funnier or more charming than I did?
I'm not sure I really liked the ending. I'm guessing it was supposed to make me think, but honestly, other parts of the story did that quite a bit better. It was a philosophical book and very thoughtful in places (perhaps that is why I didn't laugh as much as I had hoped?), but the ending just sort of...happened and then the book was over.
All in all, I'm not quite sure why it gets as much acclaim as it does. It's not a bad book by any means, and it certainly has some high points and some thoughtful moments, but whatever makes it so beloved by so many people seems to have passed me by completely.