We had lunch there during our first day at the MK during our stay last August, an 11:30 ressie.
If I have to generalize, the experience overall (food, service, atmosphere) was somewhere between a B and a B+. We arrived about five minutes early for our ressie, and were seated a little after 11:30 (despite the fact the restaurant was at best one third full at the time, the CMs didn't seem to be in any particular rush to get us or others seated).
The place has that generic Main Street U.S.A. "turn of the century" feel, and the menu is a basic range of standard sandwiches (hamburgers, clubs, reubens, etc.) I had the reuben, with a side of german potato salad. Both were good. Service was like that at just about every other table service restaurant I've ever eaten at in a Disney Park: friendly but slightly on the slow side (we didn't get out of there until almost 12:30).
Looking back, the Plaza was a nice respite from the crowds on a hot summer day, but it certainly didn't make the cut to get on my family's short list of favorites for return visits.