I've been a T-Mobile customer for 10+ years and have always been pretty satisfied with the coverage. However, when i'm in the US i'm primarily only in LA or Miami and never in rural areas.
Looks like its starts for new customers on 10/21/2013 and for existing customers on 10/31/2013.
Naturally, i'm an existing customer who is out of the country from 10/18 until 10/31....