FWIW, I love the early "Witches" and "Watch" books, but I'm finding his later books a bit... samey. All of his oddly-named male protagonists just blend into one in my memory, and though I laugh at the jokes, by the time I reach the end I've pretty much forgotten the story.
For the uninitiated:
* "Rincewind" series (Colour of Magic, Light Fantastic, etc) - parody of the fantasy genre (CoM in particular is an overt parody of Anne McCaffrey, Fritz Leiber, Robert E Howard and others)
* "Witches" series - lots of fairytale/Shakespeare references/parodies
* "Watch" series - medieval hard-boiled detective fiction, more satirical
* "Tiffany Aching" series (Wee Free Men, etc) - arguably YA, with much younger protagonists
Other early books tend to be more general humorous fantasy, whereas recent ones tend to target and satirise specific modern phenomena: football, mass communications, etc. That's a generalisation, of course, but it's my overall feeling of his development in the past quarter century.
"Nightwatch" is one of my favourites, but maybe I'm biased, as I lean towards this book's blend of light-hearted action and gritty nastiness in my own writing