I have to be honest here; this is one of the few later Pratchett novels I cannot be enthusiastic about. It displays his inimitable style well. The humor level is right up there with his best. The characters are very well-drawn and interesting, particularly Mightily Oats, a young Omnian preacher who displays a lot of good sense under his dithering exterior. And, of course, the message of the book is very well done.
The problem is that Pratchett already wrote this book, and it's called Lords and Ladies. You have the kingdom of Lancre, check, a threat to the country's young reigning couple, check, a threat by supernatural forces who see other races as lesser, specifically prey, check, stood up to by the inventiveness of the witches, particularly Granny Weatherwax, check. This isn't nearly as bad as it is with, say, Tolkien clones; Carpe Jugulum is indeed different enough to qualify as a different story. And the message— that people are not things and should not be treated as such— is something that, sadly, needs to be reiterated constantly.
But somehow, I kept feeling as though I'd been told this before. Somehow, this book may be different from Lords and Ladies, but it just isn't different enough.