We went last year without the dining plan and just returned with the dining plan. Let me just say that we spent about the same amount of money, but we really enjoyed this trip so much more. (Last year, it was mostly counter meals and only a few sit-down meals).
We went to the food court our first night there, and just for four hamburger combos it was $54.00. We ate and ate and ate more. We don't usually eat appetizers and desserts at home, but this was our vacation. I think we saved $400-$500 being on the dining plan.
Now, if we were paying OOP, we wouldn't order the appetizer and dessert, so technically we probably didn't really save any money, but it was so nice not worrying about money and did we stick with our budget for the day. I could order what I wanted and I didn't have to worry if the kids ordered something that I knew they would never eat. The kids tried new things that they would never try at home and I would never want to pay for since I knew it wouldn't be eaten.
So, for me, it wasn't about saving money. I knew I could do that on my own. It was about convenience and enjoyability. I enjoyed myself so much more this year than last year. The kids enjoyed themselves because I let them have whatever they "think" they wanted.
Also, don't forget that the dining plan includes tip and tax. For $100 meal, that's $18.00 tip and $6.50 tax.