I'm currently rereading the book, and a couple of things strike me.
First, I think this is by far the darkest of the Discworld books. Still quite funny, of course (the "Poosticks" line had me cackling for a good five minutes), but still very very dark. I think it's partly thematic, and partly because the characters traditionally used for comic relief (Fred and Nobby spring to mind, as do Cheery and Constable Visit) are either much more serious or much less involved this time around. I think it's appropriate, considering the subject matter, and the City Watch books tend to be a bit more thoughtful anyway. (I'd say the previous darkness champ would be <i>Night Watch</i>, although it had Leggy, Snouty, and the History Monks to offset it.)
All in all, I think that's a good thing. It's nice to see that Pterry doesn't have to be riotously funny <i>all</i> the time.
I'm a bit disappointed on one score, though. I don't think the Summoning Dark is nearly as effective as The Beast. Vimes isn't even aware of the Dark's presence until it's no longer a threat, and thus never has to come to terms with it. The Beast is such a useful tool for him - both adversary and ally - that it's a pity not to see the idea explored a little more.
I actually expected the Dark's downfall to be The Beast - a sort of a "you're on my turf, bub" type of confrontation. Or if the Dark had corrupted the Beast, or allied with it, I think it could have been more of a threat. As it is, it never had a chance, and we knew it.
All in all, a very good book. It definitely lives up to Pratchett's high standards. A bit of a letdown after the superb <i>Night Watch</i> (my favorite of the lot), but still full of good characterization, fun wordplay, laugh-out-loud jokes, thought-provoking ideas, and even a strong moral that isn't beaten into our heads. Two thumbs way, way up.
_________________"Vox populi? Vox humbug!"
- William Tecumseh Sherman