Darn impressive even if not always doubling speed. I wouldn't expect to get 400 megabits at Broadway and 42nd Street at 6 p.m. Some people, in just the right spot, will see speeds close to that. Speeds of 50-150 megabits will be common many places, although those at the edge of cell or through a thick wall the speeds will go down to under 10 megabits.
CTO Neville Ray has done an extraordinary job keeping up with the technology despite a capital budget of under $5B, not much for a U.S. sized network. He's also extended coverage to 311M pops, about 98%, almost matching Verizon. (Independent testing of coverage is often lower than the telco claims, but I have no reason to believe T-Mobile lies more than Verizon or AT&T.)
The moves to 256 QAM (downstream) and 64 QAM (upstream) won't affect many people because so few phones support them. QAM measures how many bits you send per hertz. Higher QAM encode more data. The analog components and digital signal processing in new phones are improved, allowing more signal to get through. 256 QAM will be common in new phones in a few years.
Ray should not have signed his name to the claim, "delivers a massive 2x speed boost to customers."