Hitchhiking is a feasible way to travel independently around Namibia, provided that you're patient and don't have a tight schedule to keep. It is certainly one of the best ways to meet people, and can be speedy and cheap. How fast you get lifts is determined by how much traffic goes your way, where you stand, and how you dress. Some of the gravel roads have very little traffic, and you will wait days for even a single car to pass. The important part is to set off with enough food and (especially) water to be able to wait for this long, or to choose very carefully the lift that you take, and where they leave you.
For the sake of courtesy, and those who come after you, it's important not to abuse people's kindness. Offer to help with the cost of fuel (most people will refuse anyhow) or pay for some cold drinks on the way. Listen patiently to your host's views and, if you choose to differ, do so courteously – after all, you came to Namibia to learn about a different country.