Thank you for your suggestions and opinions. We wound up eating at the 600 Watergate restaurant across the street from the KC. It was moderately priced and our meals were worth what we paid. Each of us had a 3-course dinner (salad or soup or appetizer + entre + dessert) for $38. I had Caesar salad + bouillabaisse + ice cream and my wife had house salad + salmon + chocolate mousse. There was more than enough to eat and we were well-satisfied with our choices. Service was very good and there were quite a few people having dinner at tables and at the bar. As a sidebar, I should note that parking at 600 Watergate is $10 as compared with $17 at the KC.
Since you're used the phrase "any good" the answer is yes. But you've reached the threshold.
If you eat there, your best choice is to assemble a salad from the fresh salad buffet in the middle of the area. (Protein choices are part of the salad offerings.They used to offer a whole poached salmon for patrons to take as much as they wanted. The last time we dined at the cafe, the salmon was not offered.) In cold weather, you might want to team the salad with some usually reliable soup.
Another strategy is to select something from the station where a chef prepares two entree options on-demand. The problem with this option is that often this station remains unattended until the crowd begins to build and long waits may be involved. If you've arrived early to maximize the ease of locating a seat and minimize the time you're deaing with the dining room at its noisiest, you won't have this option.
There's a station at the end of the area offering two already-prepared entrees and side dishes. That food definitely falls into the "any good" category. There's an equivalent station offering pizzas, pasta, and soup. I don't think we've ordered anything other than soup from this station.
One last issue: The cost is far more expensive than the quality of the food and the cafeteria ambiance. You're paying for location and convenience.
We've eaten several times at the Kennedy Center's casual restaurant and liked the food - not sure if it's called the KC Cafe (but most likely it is--it's not the expensive one :) We prefer to eat a light meal before a performance and we like walking around the buffets to make our selection of foods.