This thread is a couple months old so I just wanted to confirm some things now that I've actually played the game a fair amount:
The Zulu's overrun advantage is 4:1, not 3:1 as others claimed. The main advantage of the Zulus overrun ability, from my experiences anyway, is that Zulu Impi armies can clean up barbarian villages in one turn. This gives the Zulus a major advantage in getting barbarian villages not only from their warriors ability to move two tiles in one turn, but also from their ability to attack barbarians more than one time per turn. In addition, the advantage is also that they don't have to worry about healing from fights where they're heavily favored.