T-Mobile announced its new "Test Drive" initiative a couple of months back. If you opt to participate, T-Mobile will send you an iPhone 5s so that you can test the carrier's network before you decide if you want to make a switch. The company confirmed with Re/Code recently that it plans to upgrade its Test Drive program to eventually include the iPhone 6 instead of providing people with Apple's last-generation smartphone.
"We don't have a definitive timeline, but we do intend to switch over in future," T-Mobile told Re/Code. "There are a variety of factors that affect that timeline, [including] product availability." T-Mobile also confirmed that it will begin rolling out Apple's brand new iOS 8 operating system to the existing iPhone 5s Test Drive units so that the software experience is as up-to-date as possible. Quality assurance testing will begin soon and the software update may hit Test Drive iPhone 5s units next week, mere days after it launches for everyone on Sept. 17.
T-Mobile recently revealed that it will provide routers to customers that will boost the carrier's signal in poor coverage areas, like in the basement of your house, as part of its Un-carrier 7.0 announcement. It doesn't appear that that will also ship as part of T-Mobile's Test Drive package, but considering the iPhone 6 and iPhone 6 Plus support VoLTE and Wi-Fi calling, T-Mobile should consider adding that to the package in the future.