Actually, au also has very good coverage. It's comparable to Docomo, and much better than the other two companies when it comes to the more rural areas.

They're not the cheapest option, but their connections are 4G LTE, so if you hate the google map load time like I do, they are pretty good for short-term rentals:

NINJA WiFi (lesser known, but decent value for short visits)
http://ninjawifi.com/