I've had my G Pro since day one and it's not unlocked from AT&T. I just recently signed up for StraigtTalk and will be canceling AT&T. Popped in the ST sim and entered their APN settings that the site said and I get 15mb down and 2.5 up. I haven't been capped on data either and I'm pushing 11Gb now in usage.
Future Phone: LG Google Nexus 5
Current Phones: Samsung Galaxy Note 3 AT&T[Rooted], LG Optimus G Pro AT&T[Liquid Smooth]
Past: LG Optimus G AT&T, LG Google Nexus 4, Samsung Galaxy S3 AT&T, Samsung GS2 AT&T, Sony Xperia ion AT&T, Samsung Infuse AT&T(First Android Phone), All iPhones before that.