"Snuff" is well below the usual quality of the series. The humor is seldom very good, the scenario is less than exciting (the events aboard the "Fanny" are the one exception) and, all in all, the story feels uninspired.
The first 100-120 pages are fairly boring. There's an unbelievable number of repetitive jokes about Vimes being henpecked by his wife, which can actually give the impression that their marriage is not truly happy (fortunately, the other two thirds of the book do a good job of reversing that impression).
Racism and xenophobia are an interesting theme, and books such as "Jingo" have done a very good job of dealing with it. But, unlike several books of the series (including some of the very best ones), "Snuff" doesn't really make the reader think. The goblins are little more than a plot device (rescuing them is what moves the story forward) : their introduction is rushed, their way of life is little described and while the Unggue is the focus of the first two pages, it often seems forgotten afterwards. The fact that the way goblin are viewed is changed because they happen to number a musical genius among them is rather cheap.
Vimes has become a rather static character. His inner darkness is an interesting element of his personality, but it's been broached much more subtly in previous books. The main problem with him in "Snuff", however, is that he simply never faces a serious challenge : he guesses the truth easily, convinces people to side with him easily, defeat obstacles easily... It's hard for a novel to be exciting when the protagonist manages so easily to always get his way.