We tip 20% and up, at all types of restaurants. At the buffets, I think the waiters/servers work just as hard because they are constantly clearing away the dirty plates every time you get up to get more food. (Restaurants do NOT want you taking a dirty plate back up to the buffet line!) So really, I think they have just as many, if not more, plates to clear than at a sit-down restaurant.
We're big tippers anyway, because we appreciate how hard it is to wait on people. It's a job I've never had, and hope to never have. lol