You should be as vigilant in Johannesburg regarding your safety as you would be in any other major city in the world. 

Admittedly, crime has been pretty bad there in past years with numerous hi-jackings and car-jackings, as well as more serious crimes, but the police force has recently announced a massive reduction in crime.

You wouldn't venture out at night on your own walking the streets and this applies to most areas in Johannesburg.  Driving a car presents no problems but you would need to make sure you know where you are going.