What's important to note is that T-Mobile is quickly becoming a company that delivers a great product and is customer-centric. And when the incumbents start playing catch up (AT&T in particular) you know they're making the right moves.
I've been with Verizon for a few years and I can't say their service is what it's marketed to be. Data is fast but the inability to simultaneously do data and voice is just idiotic. The monthly fees are also totally insane. I pay $140 / month for a single data line (iPhone). In contrast to Verizon, with T-Mobile you get unlimited voice and data for a little over $70, no contract and stellar customer service. I called T-Mobile to switch last month and with a little negotiating, they sent me a _free_ iPhone 5S. No contract. No bullshit.
T-Mobile's latest announcement of up to $650 in credits for switching over will increase their subscriber base by millions of new users, no question. It's bold, aggressive and customer-centric.
I can't personally speak on AT&T's network but we all know it's one of the worst companies in the world to deal with. Their service is spotty for many, many users. Their customer service is non-existent. Their data practices are borderline unethical (with hidden data caps, throttling, etc). AT&T is just an awful company. I'm stuck with their uVerse Internet service and it's just a disaster. Unfortunately, many of us have no other choice but their lousy broadband product. Let's not even talk about Time Warner.
T-Mobile's coverage, from what I have experienced, is stellar. Up and down the California coast. Mexico. Europe. NYC. Seattle. I'm always at 4+ bars. Data is fast (~18Mbps downstream). International roaming is perhaps one of the most impressive features of TMO. I remember paying $20 / MB with Verizon and that's just absolutely criminal. And they can get away with it because many customers aren't even aware of this until they get the bill.
Legere is right - the mobile landscape is just horrendous and it's mainly because of AT&T and Verizon. I hope T-Mobile continues this push towards making customers happy and simply offering better products. Their bottom line will be the ultimate proof of how this is working out for them - and from what I'm seeing, their new subscriber numbers are growing exponentially. You could say they're disrupting the industry. And that's rare from these monoliths.