I've come to add my data points to this. Would it be possible to add a link to this thread Wiki where people may comment on usage in specific locations? Or an idea similar to this?
In Japan, I used the plan without issue in Narita, Ikebukuro, Omotesando, Yokohama, Ueno, and all parts in between.
However, in Guangzhou, I had 3g in the airport, EDGE coverage 2 stops away from the airport and none in Guangzhou itself. I was primarily located near the East Railway station but spent a fair amount of time near Zhujiang new town. Although I considered using China mobile, I was spooked as my welcome text from T-mo only mentioned China Unicom so I used WiFi for the most part while there.
These may have been mentioned in other posts but a search revealed nothing and I don't care enough to read 26 pages of posts for someone else with the same experiences.