I've been twice, can't recommend it enough. We went to a game reserve on the Eastern Cape and another in Madikwe in the north because they were both malaria free which meant we didn't need anti malarial drugs (I was on other drugs at the time and was keen to avoid taking more).
You aren't actually that far away from civilisation - if you needed it most of them can fly you out within hours. From memory, Madikwe is about a 4 hour drive from Johannesburg, but only an hour or so by plane, and they have small aircraft flying in and out daily. And there are smaller towns much closer.
Just make sure you're prepared - I never travel without plenty of spares, split between me and the husband, including lots of Glucogel and Glucagon so we can cope with even major hypos by ourselves.
Make sure you can be flexible with meal times etc - safaris tend to start very early in the morning with a quick snack before you go out and then brunch about 11am ish when you return from the morning game drive. Then afternoon tea before the evening game drive and dinner when you get back after dark. As long as you're comfortable being flexible, it's fine.
I'd say go for it, it's the most amazing experience. Our biggest problem was the food, it was amazing and because safari is generally full board there was an awful lot of it. Self discipline went out of the window!
