To me, Ubuntu is a very difficult concept to explain. It means that all people are only there because of other people and therefore if you live for yourself, you do not really live. Doing things for other people, like taking food to people who do not have any or sharing clothes, blankets or anything else with people who need it, is not seen as an act of kindness, but rather a kind of duty. If someone in the community suffers while others do not, it is because the community is failing. Ubuntu means that one can not prosper while another suffers.
I would, however, not say that Ubuntu replaces kindness in South Africa. I am a South African, but of Western origins rather than Bantu origins. For me, as for most of the people I grew up with, kindness has always been something that is seen and appreciated. I had only really started to understand the concept of Ubuntu when I started teaching at a Xhosa school this year. Where I come from, Ubuntu is a word we know, but not a concept that is generally understood or believed in.