I've used T-Mobile on perhaps a dozen trips to Japan, and I'd say it's perfectly adequate for checking email/reading FT/maps/face time voice. There's generally pretty good signal almost everywhere, and I'm not just talking central Tokyo, but places like Kushiro or Ube and the trains that get there.

In general, I think the T-Mobile plan is one of the best travel values out there; I'm paying some $19 a month on a shared plan, and generally have slow data available anytime I step off a plane. Some places are faster than others, but I never had no service at all (the only exception being Macau, where the T-Mobile is severely broken). Obviously the countries I visit more frequently or stay for an extended time, I get a local card. In Japan, I'm fine with T-Mobile and the hotspots that are available in lots of places.