I'm afraid Pratchett is almost invariably a hit for me, so I can't speak as to whether you'd enjoy it.

I like most of his. Just occasionally one doesn't grab me. Going Postal and the vampire one didn't.

Going Postal was a later work where his illness was starting to show, I think. I enjoyed it but it was a bit of a departure from his other work. The vampire one was fun but again not his best work.

Can't speak to "Going Postal" (yet), but if by "the vampire one" you mean "Carpe Jugulum" (vampires trying to depose Verence and Magrat, all the while Granny Weatherwax is on strike and nowhere to be found for the first half of the book) ... that isn't a favorite of mine, either.

Yes, that one. Couldn't think of the title at the time.