I have the same phone, the SGH-T999, T-mobile branded. It worked fine for a month but I ran into problems with the seller on eBay (doesn't appear that it is a truly unlocked phone, they are also not the authorized user so T-mobile can't send them the "SIM Subsidiary unlock code"; I'm working with T-mobile now, faxed them my receipt and they said they would send the unlock code within 10-14 days).
When it was working this phone screamed on the network! I'm in the process of switching over from Sprint (was never able to get 4G at home with Sprint; I get 4G in the garage with Solavei) and really want to get this phone working again.