B-mobile has revamped their visitor SIMs. The original product is no longer available. Instead, there are now two prepaid, data-only SIM cards. Both cost ¥3,980 and are valid for 14-days, with a choice between regular and micro sizes.
These SIMs are delivered preactivated with English support (including the webpage).
The "14-day prepaid" has unlimited data capped at 300 kbps, and the "1 GB prepaid" allows full speed (best effort) up to 1 GB. It should be noted that the 300 kbps version will not allow sustained streaming, so it won't work with VoIP, Skype, youtube and the like.
Because of this, I recommend the 1 GB flat rate SIM.
Also, the 300 kbps SIMs often have high latency, meaning it can take a little time for a connection to be completed. All of the "U300" products have this issue. The 1GB version will provide a much more responsive internet experience. However, these are not a wide-open connection to the internet with a public IP address. These connect through a proxy and there is some amount of traffic shaping. Most people find that they can use VoIP like Skype with little issues.
While the 1GB version is marketed as "perfect for Skype," I confirmed that the SIM is technically equivalent to the standard 1 GB Flat Rate SIM and has no additional "Quality of Service" provisions specifically for Skype.
Non-visitor data-only SIMs also an option
See here for a list of SIMs that can also be used by visitors. Voice SIMs are by law limited to residents.
There is no reason why a tourist cannot use a standard, non-visitor SIM while temporarily in Japan. These are less expensive and the prepaid charges are good for 30 days. However, these require access to a Japanese mobile phone to activate the SIM. An English activation menu is available. These can be found at large electronic stores, such as Yodobashi Akiba. The 30-day U300 is not available in retail stores, but can often be found in the Akihabara gray market shops. It can be delivered to a hotel room.
The increase in price for the visitor SIMs may be worthwhile for the decreased hassle. Some may note that the there has been a substantial price increase on the visitor SIMs relative to the price of the original, with the price per day increasing from ¥100 to nearly ¥300. All I have to say about that is JCI made a mistake in pricing the original. Why would they offer it at the exact same price as the regular SIM when the market would obviously allow a higher price point?