We went as a party of four to get a quick lunch at midday on a Monday in February. The place was quiet and uncrowded. It is a self service cafeteria, but there was no line/queue the day we went. We stood while the cook grilled our burgers (one of them a bean-burger). There was a good selection of pickles, cheeses and other fillings to choose from, all fresh and nicely presented. The seating area is light an airy with great views over the Potomac. Afterwards we went for stroll on the large terrace.
We attend Kennedy Center performances regularly and eat here most times. Food is pricey but you get a healthy portion. Most times hot buffet food has been cold, but we keep trying. Crowded and noisy dining room but there are tables in the foyer and it is quieter there. Given the choices, this is the place we choose to eat before a performance for the convenience. We come for the entertainment -- not necessarily the food. Tonight the wine bar ran out of glasses so they are offering $10 glass of wine in plastic cups (similar to ballpark beer glasses).
When attending an evening performance at Kennedy Center, particularly for an early show, the KC Cafe provides a satisfactory place to grab some food. You can serve yourself from a large selection of food, ranging from a salad bar to sandwiches to several hot dishes served rom a hot stand. There are also a couple of overpriced wines. It provides an acceptable meal but not a dining experience.