Also consider that you have to get to the bus station to start with since Johannesburg Park Station isn't yet connected with the Gautrain service from the airport. I don't know what the surrounding area is like nowadays but I wouldn't be particularly keen to find out (or any other bus station in/near the city itself), especially since the services tend to leave either very early in the morning or late at night.

There's also a bus station used by Intercape and Greyhound at Midrand (halfway to Pretoria) which I think is connected to the Gautrain.

But flying will be easier, quicker and have less chance of immigration hassle.