I thought his later books were better. I really didn't get on with anything containing Rincewind, but I like the Tiffany Aching books and enjoyed the later Sam Grimes and the Moist Von Lipwig books. I think the first one that really clicked for me though was Mort.
Sam Vimes is my favourite character in the series. In all likelihood due to our professional outlook. The passages about Sam standing in doorways, collar turned up against the rain, knowing exactly where he was in the city due to the feel of the cobbles through the thin soles of his boots etc. I was probably at my most professionally content during the early years of my policemanship, 3- 4 a.m. on a miserable winter morning, foot patrol in the town centre rattling doors, gannex mac collar turned up to stop the drips off the brim of my helmet rolling down my neck, hiding in shop doorways, puffing on a crafty fag or watching the early morning ner do wells as they prowled for something illegal to do. Happy days.
Many of the successful characters are part of a double act- Esme Weatherwax and Gytha Ogg, Vimes and The Patrician, Carrot and Angua, Fred Colon and Nobby Nobs. The interplay between them is often a joy. Simple but funny.