While this is nice, especially for short-term and infrequent visitors, the extremely low 300kbps bandwidth limit is somewhat disturbing given the price given what you can get directly from DoCoMo.

For between 1,000JPY and 5,985JPY a month, depending on your usage, you can get a full 7.2/5.7mbps unlimited data service:
http://www.nttdocomo.co.jp/service/d...lan/index.html

The usual terms and conditions apply; that is, a two year contract, alien registration card or 外国人登録原票記載事項証明書 (I'm not sure how to translate this exactly, but it's the little piece of paper you can get from the ward office to prove you are registered).

Still, if you travel to Japan frequently (or live there!), I'd probably consider this over the b-mobile device.