I haven't volunteered with that particular organisation however I have done international medical aid in Asia and Africa and I have also been to Cape Town so I wonder if I have any advice to offer.
First off, Cape Town is a beautiful city, and, in the right parts, feels just like my home town of Sydney, Australia. However just on the edges of the "nice" parts are living conditions that we do not have here in Sydney. Certainly however, travelling in Africa is certainly very different to travelling in Europe. (Having done both, I can attest to the significant differences!)
In terms of choosing an organisation I feel that you need to be very careful. Have a look into overall aims of the organisation. They may not state these clearly but if they are charging you large sums to volunteer or are short term, fly in fly out missions most certainly raise your level of suspicion.
I think it's very difficult to be ethical about volunteering in developing countries but it's very important to look at whom the benefit is being provided to by the experience. Is it the local community or is it the volunteer? Have a look at Unite for Sight's suggestions for ethical and sustainable volunteer/medical aid in developing countries.