We ate at the Plaza on our honeymoon trip last month and loved the experience.
We arrived about five minutes before our ADR; there was very little wait before we were seated and we were shown to a table sprinkled with confetti.
Our waitress was very helpful (making suggestions since this was our first time eating here) and very friendly. She was also accomodating - DH made a couple of changes to his sandwich order and I ordered a cherry coke, which they didn't technically have but she had them make one for me using cherry syrup they use on the icecream. There was no problem with any requests we had and throughout the meal we didn't have to ask for refills of anything.
I ordered the vegetarian sandwich and DH had the cheese-steak sandwich. Both were very good and very filling. For dessert we split the apple charlotte. Everything tasted delicious and the portions were large.
I certainly wouldn't say that the service was slow, but the atmosphere is a bit different from other places in the park. The service was prompt, but not at all rushed. We found it to be more relaxed and a nice way to take a brief break. We would definitely eat here again.