Let's hear what real T-Mobile customers say (names are fictional):
"My iPhone 6 went to No Service and started searching for other networks... My Avant, however, was connected to a quality band 12 signal."
"For what I do, band 12 is the difference between no service and LTE."
"We just had it turned on a few days ago here in Denver, and pretty much every location where I had B4 signal drop, I get solid, usable B12 now on my Nexus 6."
"In Houston, I always ran into dead spots in the past (my parents home was one of them). Now I'm getting band 12 in all of those spots."
"B12 has drastically improved spotty areas for me including buildings. It's like a mass turn on of new towers."
"The bad news it that I still have a terrible signal/speed at home band 12 or not. :("
"Last week I stayed in a Trump Hotel. Band 12 was everywhere. The management was furious!"
As you can see by the last two quotes your mileage may vary. Band 12 is not available everywhere and not all phones support it. For details see the 700 MHz map (only purple towers are confirmed locations) and compatible devices.