We opted for the Airport Pickup Service, which was simple enough, the airport post office was even open on a Sunday! Just make sure you pick the correct airport terminal from which you arrive at, Narita Airport kindly provides a list of various airlines here.
Apart from the inital setup, the 3G was fast, consistent and we had great coverage in Tokyo and Kyoto.
However, I found the b-mobile user manual setup instuctions quite inadquate so here's a more helpful guide for iPhone4S (iOS6.1.3).
To edit APN settings on an iphone4S,
Go to Settings > General > Mobile Data (scoll to the bottom) > Mobile Date Network
Enter the following information:
APN : bmobile.ne.jp
Username : bmobile@fr
Password : bmobile
After that, you should be connected to JP Docomo's 3G. やった！Yatta!
Do stick to the list of supported devices as provided by b-mobile Japan! We tried to use H's unlisted HTC Desire S to connect initally (as we find google maps app works best on android systems) but we simply could not connect until we switched to an iphone4S. Thankfully, the b-mobile Visitor SIM supports tethering!
Other free wifi options I found were:
1) NTT East's Free Wifi Japan for tourists.
A free option if you're going to be in central Tokyo for less than 14 days. We were told to go to Akihabara to pick the free wifi card but unfortunately, ran out of time to make it out there and try it.
Let me know if you try this service! Would love to know how many access points there are and how well it connects.
2) Free Wifi at Starbucks Japan
Register at http://starbucks.wi2.co.jp/pc/index_en.html
I would register for it before leaving for Japan as you need to complete registeration within 24 hours via a link sent through a confirmation email. Oh, and please do buy a coffee at least for the services!