I was in Nairobi together with a big group, and we spent one day in the slums of Kibera. We visited a school driven by the Children of Kibera foundation and was guided through parts of the slum with a guide. As a foreigner, you should never walk in the slum without a local guide, as this can be very dangerous. Listen to your guide, and be discreet when taking pictures.
Even though a walk in the slum isn't a pleasant experience, I think every tourist coming from a rich country should visit the slum. It gives you a new perspective of life, and even though you might forget some of it when you go home, it will have taught you something. While you are in the slum, I will highly recommend a visit to the Children of Kibera school - it's an amazing project that gives children of poor families an opportunity to go to school, and the best students can even attend college in the USA. Coming from the 2nd largest slum in Africa, that is an amazing opportunity for the children there. Also, by visiting, you will highlight the children's day and learn how a school in the slum looks like.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.