I've eaten there just once -- for dinner -- and would recommend it to anyone paying out of pocket. It was our arrival day during Easter break in 2005, and we were able to walk up at around 5:00pm and get a reservation for the six of us for just half an hour later. We grabbed an outdoor table just outside the restaurant and people-watched along Main Street until it was time to dine.
(Can I just take a moment to say once again how amazingly blessed we were on that trip? Walk ups and free outdoor tables along Main Street during Easter break? At dinnertime? On a Saturday in the Magic Kingdom? It was my parents, me and DH, and my sister and BIL. Mom was in failing health and we all knew it would be our last Disney World vacation with her. It was like God just paved the way for us, wherever we went that week.)
Okay, back to the Plaza. We were called in a few minutes before our reservation and had a corner table in this sunlit, intimate little eatery that felt like a Victorian-esque tearoom. Everyone ordered a sandwich or a salad. I had a salad and recall that the ingredients were crisp and fresh, and DH had some kind of club sandwich that he enjoyed. I don't recall what everyone else ordered, but I remember that everybody was satisfied with their meal (other than someone's french fries not being hot enough for their liking), and we all felt that our money was well spent. It was by far the cheapest table service meal we had that week, barely costing more than counter-service. We were not on the dining plan, and for what it's worth, I've seen The Plaza repeatedly billed by others as the best table-service value in the Magic Kingdom if you are paying out of pocket.