This sort of drive-in is a vanishing breed. Rather than going in to eat, or pulling through a window in a hurry, you pull up to the box and try and get everyone to look at a tiny menu. Then someone brings it out to you and you figure out how everyone will get what they ordered and eat it in the tiny space of your vehicle. Awesome, and that is just the experience.
As to the food? Very respectable, especially if you stick with the burgers. Perhaps even some of the best in town, keeping in mind that these are not gourmet by any stretch of the imagination. Just straight up simple hamburgers, and that's the way they should be done sometimes.
While other South 21 locations (notably the one in Matthews) take credit cards, not sure if this one does - at the very least, you're making someone walk a good distance. You might want to take cash.