Just found your thread by searching since I'm looking at a Greenzone bike on craigslist right now. Basically, the $300 model from Greenzone (currently on sale for 200 by the way) is made of pretty low-end steel and has absolute bottom of the barrel components. It weighs a ton, which will make you unhappy on hills, and the reliability of the parts is likely to cause you headaches. Derailer going out of tune constantly, brakes doing the same and generally underperforming, wheels always out of true, that kinda thing.

That said, I'm pretty inclined to pick up the one I'm looking at if I can get the guy down to about $100, because I just want it for a beater/commuter bike for riding around the city so I don't have to leave my nicer bike locked outside exposed to the elements, thieves and vandals. If all you're doing is cruising around town (especially a relatively flat city like where I live) it might be a good investment.