I got some sugar'd Coke in Flagstaff a few years ago. It's a smoother, less biting sort of taste, although I'd need to try it again to give a clearer description. I wouldn't call it a massive difference, though.

Even though Coke won't do it for whatever reason, Pepsi has been selling fairly large quantities of their product with real sugar recently. There's a definite difference there: a distinct increase in perceived sweetness compared to the syrup'd version. Sometimes I like that, but other times I don't.