Yes Africia is complelety amazing- the feeling you get when you see the animals and people there is just fantastic and the whole experience is just something so far out of your comfort zone as well.
I did a 4 week trip overland from Vic Falls to Cape Town last summer with Africa Trails. It was actually the last leg a bigger trip which went either from Nairobi to Cape Town (8 weeks) or Nairobi out to the Gorillas in Uganda and then down to Cape Town (10 weeks). My leg of the trip was brilliant and judging by the tales of our new friends who had done the whole journey if you had the time and money its definalty worth doing as much of it as you can. The good thing is that African Trails is in itself its own company and you can book on their website, they don't outsource like On the Go, Topdeck etc. They have a mixture of African, Aussie and British staff/drivers in Africa and heaps of locals lend a hand on the way. They are a bit cheaper than some of the other tours-mainly cause you have to cook as a group and have an older style open side truck which is a bit less comfortable but I think kinda more fun.
I would recommend a tour of Africa rather than going yourself- it's a big place to get around and you'll get ripped off with a lot of stuff if you don't haggle properly for taxis, rooms, campsites, buses etc. The tour is just so much eaiser plus you get organised safaris etc.
If you do pick an overland trip be prepared to be roughing it quite a bit. We often went without a shower or electricity for several days and spent a few nights in bush camps on the side of the road- but it was actually nice to get away from it all for a while. That said there are still some amazing campsites in the bottom part of Africa with great bars, huge campfires, swimming pools and cheap beer :)
Good luck and have a fantastic time