This is by no means the best of the Watch novels ... but even an average Pratchett novel is a thing of beauty. By turns deadly serious and laugh-out-loud funny, the book has only three weaknesses (which other reviewers have touched upon).
First, most of the Watch characters get barely anything to do. Second, the Patrician is way out of character. And finally, the ending is pretty weak.
Nevertheless, this is a still a great and very entertaining read. I wouldn't recommend it as a starting point, though, as Guards! Guards! is a better introduction, Men at Arms is funnier, and Feet of Clay is a better mystery.