Going Postal is my second-favorite Discworld book, after Night Watch, but I think Going Postal is definitely the funnier of those two.
I recently read The Lies of Locke Lamora as well, and I really enjoyed it. Sometimes fantasy novels feel like their entire world was invented just for the sake of the story, but with this one I really got the feel that the world was there, and the story just happened to take place inside it. If that makes any sense. I have not read the sequel, but honestly, LoLL has a satisfying ending on its own. I'm glad there will be more, but even if there weren't, I wouldn't let that stop you from reading what's there.