On our Round the World Trip we had to rely on paper maps as well as a GPS since Garmin doesn't have maps for all the countries we were traveling in, and we didn't have the time to find third party maps that would work. Well I just purchased and installed all the maps I will need for our trip to Cape Town form one source "Travel By GPS" it has all the main roads as well as some facilities.
Do you need A GPS? Well not really if you have paper maps and have time to navigate the crowded streets of a city. I rather have one than not, and I will be taking one paper map of the whole of Africa for reference.