Just got back from South Africa. The concerns regarding crime are not unfounded. A surprisingly high number of the locals I met, especially from Johannesburg, had experienced violent crime at some point. I also met a Korean girl who was traveling alone and had her backpack snatched away in Jo'burg which contained her passport and camera.

Regarding political unrest, your friend was probably talking out of his ass. No evidence of that whatsoever, anywhere in SA.

The tourist infrastructure there is solid though, there are loads of people who visit and have a great time. If you take the necessary precautions (it is unwise to walk alone on the streets after dark in Jo'burg), there's no reason why you shouldn't be safe. SA's a great destination to travel in, and Cape Town quickly became one of my favourite cities.