That's the Hillbrow Tower, a telecommunications tower and (at the time it was built, at least) one of the tallest structures in Africa. It's named for the neighborhood it's in, Hillbrow.
The Jozi skyline is dominated by two large telcom masts on either side of the downtown area, the Hillbrow Tower in the east and the Sentech Tower (also known as the SABC tower) in the west.
The building just next to the Hillbrow tower in Nomad's picture is another local landmark, a large cylindrical apartment bloc officially named the Ponte Building but colloquially called the Vodacom Tower after the neon advertisement that circles the top of it. (Once upon a time, it was a giant Coca-Cola logo, so I assume they called it the Coke building then or something.) When I first moved there it had become a virtual no-go zone, completely taken over by gangs & thugs, but apparently it's been gutted and renovated lately.
But yes, the Red Ants have moved in. One of my housemates, who's one of those crazy surfer guys, regularly goes to De Ponte to abseil down from the roof (!!). He told me that 4 years ago, they only dared to get in wearing bullet proof vests. Now the scariest things in the building are the smells and the elevators - they don't stop properly, but oscillate up and down for a while but the doors have already opened...
I briefly considered to get one of those new apartments, but the areas around De Ponte are still rather rough. If I had the money, I'd follow what is described in your linked article: buy it as investment. I'm pretty sure that within 5 years time one'd make a large return.
I remember when even the cops wouldn't go into the Ponte, and I wondered if it was even going to be possible to clear it.
Regarding your roommate's passtime, I guess I need to be thankful that I have enough excitement in my life that I don't feel even the slightest need to jump off a building in Hillbrow in my spare time.
