In comparison to a factory unlocked iPhone, are there any cons to buying a used iPhone 4S unlocked by a Gevey sim?
The big downside is that the phone isn't really unlocked.
Apple could patch the exploit that allows it to work in a firmware update. Also, you may have to re-unlock it every time you power off your phone or lose signal for more than a couple of seconds. Some people also report poor signal and battery performance with it. Theres some more info here or just google "disadvantages of Gevey Sim."