Pratchett is a very special author! Though not my absolute favourite, he does manage to combine brilliant humour with an incredibly observant social satire and deep, likeable characters.
I've read them all multiple times, and go back to them again and again. As with you, I still occasionally spot new things, or get taken afresh by a turn-of-phrase. I still laugh out loud at some points.
I think I've read the entire series at least two times. Now I tend to pick and choose - the Death books and the Watch books are my favorites.