The Gautrain is a really awesome way to get to the airport. And parking at Sandton and then taking the train takes away the lottery of the roads, especially in the current mix of rain and construction.
But the parking rates are way too high; and in some respects just doesn't make sense. For the airport, the first 24 hours costs R29, the second 24 hours cost an additional R29.50 and then for the third day onwards costs R97.50 a day. While the parking charge is less than the parkade costs at OR Tambo (R130 a day), it becomes much higher when the train ticket is taken into account. Only for the 2 day trip, is the Gautrain a cheaper option than driving to OR Tambo and parking at the parkade.
With the bus routes being rather limited (which I suspect is one of the reasons they are often empty) - the prospect of just jumping on to the Gautrain system is still a long way off. And until then, the economics of using the train if you don't live in and around Sandton, for anyhting more than a 2 day trip is going to be hard to justify.