At the Disney Restaurants they will take your drink orders, bring drinks, keep your drinks refilled, clear out use plates, bring additional flatware if needed and even discuss items on the buffet with you. And, yes, generally they will get the same tips as in full service restaurants.
However, if I am in a low-end buffet, such as CiCi's Pizza, where I get and refill my own drinks and nothing is done with my used dishes until I have left I have received no service and do not leave a tip.