We upgraded our DD5 and DD6 last year. We wanted it to be the perfect vacation and let me have whatever they wanted. The restaurants were great. Crab legs at Captain's Grill and others really made it perfect! I was surprised by how many times I was asked if they decided to order a smaller meal or a kids meal, if I wanted to pay cash in order not to use the dining credit. The buffets as well seem to have made a science out of saying....this is only 14.99, do you want to save that credit.
The more confusing part was getting into the parks. Typically, I guess children do not have to be fingerprinted. With upgrading them, they do. Therefore, we would just have them ready to go. Cast members would most of the time put them through, but occasionally wonder "why" they were adult tickets and complain that they should not have been given the option. Overall, it was absolutely perfect and I got the opinion that people do it all the time. I would probably do it even if I had to pay for the upgrade as it gave us unlimited options!