I've been in Kirstenbosch National Botanical Gardens in February 2013, as a part of a technical meeting. I work at a Rio de Janeiro Botanical Garden and I've been visiting many other botanical gardens around the world. Thus, I can say tha Kristenbosch is one of the Top 3 BGs in the world.
At the foot of the Table Mountain, the scenario is astonishing. A must see in Cape Town, definitively!