I assume it will work fine for you but there is no guarantee since its a modification of the sim tray and there are reports of some losing 3G and reception here and there because of it.
An AT&T version is what your Straight Talk wants to use and what guarantee do you have that the Gevey sim will survive an iOS 6 update? You might have to have the baseband preserved prior to updating. Is that something that you're ok with dealing in the future?
Also, take into consideration those bad ESN Sprint or Verizon iPhones aren't worth much and their resell value will be low considering there is no way to have them officially unlocked even if Sprint and Verizon decided they would start doing so in the future (not just for international use like it is now).