While it's true that you should be wary of anyone promising public 5G access in 2017, it's not as if competing carriers aren't planning true 5G networks of their own. AT&T hopes to have some form of it up and running as soon as late 2018, for instance. And no, they aren't just going to lean on millimeter wave spectrum (which has limited reach compared to lower frequencies) to make 5G a reality. As they gradually shut down their legacy networks, they'll reuse some of the newly freed airwaves for 5G service.

More than anything, T-Mobile's announcement serves as a goal post: you can expect some form of nationwide 5G to be available within 3 years. Although that's a long way off, it beats the uncertainty that has swirled around the technology until relatively recently. The big question now is whether all the major networks will be ready in a similar time frame.