rapid city growth is +50% food production basically, but it appears on the screen as the same food production, just takes 50% less time to grow. Basically, if you want to grow a city from 4pop to 5 pop, it takes 40 food normally, but with the zulu it's only 30 food, or basically what it takes others to grow from 3pop to 4pop.
This is extremely powerful after 5 techs, because that's when most people are doing the bulk of their expansion. Zulu cities can often be to 10pop early in the AD, I know their cities are usually 8pop or more by the time the BC ends. So this, coupled with republic, plus caps captured, early dominace of the map with lots of gold, and 50% gold after 14 techs is pretty good. In the modern they get half-priced riflemen, which isn't really that great, but it's still useful in its own right. But those first 4 bonuses are extremely powerful and easily puts them at or near the top of the list in my book.
It's just there's a lot of newer players that only know the impi warrior rush, and don't know when to stop spamming warriors and how to expand and tech up. You can claim so much of the map with the warrior armies, you don't even need to take a cap early in the game, you can basically choke off huge chunks of the map. Those vet impi warrior armies fortified are hard to uproot, especially if they are on forests or hills. Plus they are powerful camping out in the other's terriotory trying to snag settlers and new cities that don't have archer armies in them.
btw, why aren't you on 2kforums?