Within South Africa it's better not to use them. The reason for this is something called taxi wars where opposition take out each other as well as passengers. A second reason is the high accident rate especially on longer routes. Drivers pay owners x amount/day for the use of the vehicle so they rush from place to place to earn the maximum.
Exceptions is within Cape Town, between Pretoria and Botswana via Rustenburg, on the Garden Route as well as Cape to Windhoek.
From Zimbabwe they bypass Pretoria - you can get off at the Atterbury or City offramp. I would not carry on to Jo'burg as the area you stop in is dangerous.
Here it is called ET's - emergency taxis. They are cheap and relatively safe to use.
Pilanesberg, Kruger National Park, Lesedi, Backpackers,
Adventure Travel Africa Car Rental, 430 Reitz Street, Sunnyside, Pretoria, 0002, ZA
Word of Mouth Rules - Updated Jul 2004