It is an unlocked version of the device and it would work on the US LTE networks on both T-Mobile and AT&T. Also it would work on the Canadian LTE networks. Though the device has been priced quite high, at $620. Specially considering the fact that the device was launched in September last year.
Sony had also released the code for the software kernels on previous smartphones from the company so that the developers are able to play with their devices and customize them the way they actually want. Seems out to be good for Sony that they are letting users want they want out from their device and understanding them. Though the device is quite expensive, but still atleast the company made some efforts to make it available without any need to sign contracts.