Got my Z1c three weeks ago. While setting up on the AT&T network was seamless I am only getting HSPA+ in Houston, never LTE. But this is no surprise as ATT only uses 700 which the Z1c does not support. I will be headed over to T-Mobile soon which uses 1700/2100 which the Z1c supports.
Look at it this way. If you're like me, you know exactly why u bought a phone from the UK. It's unlocked and supports 4G LTE. So rather than hate on Sony for not catering to the madness that is the US carrier market, exercise your options.
Heck, Verizon won't even let you register an unlocked phone on their network. So I don't blame Sony. Do I love my Z1c? Yes!