I went back and forth on my rating for this book so many times. It isn't the greatest Discworld book, even the greatest Watch book. And yet there's still something of greatness in here.
First, the less-than-great. I like Vimes, immensely. But I still feel like he's getting to be a bit too much of a Batman character, if you will. Almost too smart, too powerful, too good at what he does. He's getting a little beyond belief. Here, I could still buy him, but I'm wondering if maybe he's getting honed to the point where he'd work much better as a supporting character. (Like his guest appearance in Monstrous Regiment, for example.)
The goblins were... interesting. For me, it was a pretty good bit of world-building. Invent a people that are, on the face of it, too disgusting to countenance. Then make them beautiful. It's a hard, hard thing to do, and Pratchett just barely managed it for me. And they are, after all, the overarching issue here, the question of whether or not they're people or animals. It's handled decently, or at least less anviliciously than the issue of the book has been handled in some of the previous Discworlds.
The expected characters are, for the most part, just as they ought to be. I love Sybil, and how she and Sam interact as a couple. Young Sam is now six, and has the same occasional intensity of focus I recognize from my niece of the same age. (Who has, thank everything I can thank, not decided to focus on poo, as Young Sam has.) I thought at first that Vetinari was altogether too cheerful. But then I realized that here is a man who had a very ambitious plan for his city. And it's all coming to pass, just as he planned it, and he's hardly had to do anything at all to get it done. So naturally he'd be feeling a bit chipper of late.
So, no, this isn't the best Discworld or Watch book, not by a long shot. But neither is it the worst, and it's definitely better than Unseen Academicals had been. And to be honest, even the worst of Pratchett is generally a cut above the average.