We usually tip close to 20%, but we have always had very good service. For example, the server offers coffee to go with dessert. I appreciate coffee with dessert, and not having to ask for it is even better! I don't think most people tip 20% at buffets, but for good/attentive service we do.
Last time I think the only buffet server we did not tip extra was at Boma. The guy was nice enough, just not that great at service!