I have free charging on my 2015 model S, but I don't do that many trips. The annual $ amount for me works out to only a couple of hundred dollars a year.
It sounds like a nice windfall to have free supercharging, but the actual dollar amount for me is very modest.
There was a study done that showed that there is a certain amount of discomfort most people feel when paying for anything - whether it is $5 for a hamburger, or $120 for a nice dinner for two. They went on to suggest that if you are involved with a lunch group, you'll minimize the discomfort by rotating who pays for the entire bill. That assumes that everyone's meals are about the same.
So my conclusion is that the best part of free supercharging is that you don't have to experience the discomfort of making a payment.