in Joburg, yes.
you might not have full 3g coverage in the mountains, but it always helps just having a backup plan.
mobile data is pretty cheap i think.
Sim cards are free, just register with passport. Then buy mobile data from a supplier. You should be able to get about 3gb for around $US 50 (400ZAR)
Ex SM610, FS570, WR450, Africa Twin(s)