Backpacking in Africa

I am thinking about backpacking throughout the African continent on my next trip.
Has anyone travelled throughout Africa on a low budget?

If so, what are your experiences and what places/countries are 'must-visits'?

I am especially interested in east and south Africa; Sudan, Ethiopia, Kenia etc.

 

a french couple walked from cape town to cairo, search on google 'africa trek' i saw the program, really cool they basicly lived on the kindness of others