I tipped additionally based on service. It cost me nothing, so I had no problem in tipping as I would have if I had paid for the meal if the service was phenomenal. If it wasn't, no additional tips.
Yes, I know they do get the standard tip, but I wouldn't tip 15% to a server I really liked who did a great job anyway. If they earned more, they got more. Also, it's nice to get a bit of cash for a job well done, and I understand that the included tips work like credit card tips. I figure that we tip to show appreciation. There's no appreciation of a good job shown by a flat 15% that Disney paid for us. I think it means more to have a customer thank you and tip you personally. I know for a fact that the servers who got additional tips were very happy with us for it, as we were very happy with them for making our meals enjoyable.
In particular, the Le Cellier and California Grill waiters earned every bit of that tip. We greatly enjoyed our conversations with them about Canada, the parks, things to see that they found fun, etc. The experience was as much about the food as it was enjoying the company, and these guys were great!
Our server at LTT, to be honest, didn't even deserve the 15% they got. If there was a way to remove the tip, I would have!