I really liked the first Moist von Lipwig book. The second one is ok. The third (Raising Steam) is just...bad. It's not funny, it's not anything. You can absolutely tell that Pratchett was losing his abilities.
To me it felt like he was hurrying to kind of tie a bow on Discworld and settle at least some of the longer-term plot threads he'd been playing with, similar to The Shepherd's Crown (which is better). The train theme of the book felt apropos, because he was loading a bunch of his characters on the express train to a reasonably happy ending.