Currently it's easier to rent a mifi device, and possibly a phone, rather than try to get a SIM card for Japan. I was researching quite a bit for my trip this past week and I decided a mifi device would be the most economical and easiest. There are a few companies that rents them and can deliver to the airport or your hotel/address.

I originally ordered through Japan-Wireless (www.japan-wireless.com) as they were the cheapest for me. I submitted my order to be delivered to my hotel and got the confirmation email which said the device would be delivered the day before so that it would be ready by the time I got there. However once I checked-in and asked if there was anything delivered for me, there was no record of anything from Japan Wireless. I got ahold of them and they told me they checked with Japan Post and was told it was supposed to have been delivered that day, but somehow it wasn't going to be delivered until the next night. Since this trip was only 2 and a half days, that would've meant over half my trip gone by the time the device was finally delivered.

In the meantime I was able to find a company that could rent in-store in Tokyo that was close to my hotel called Pupuru (http://www.pupuru.com/en/). They happened to be running a promotion through the end of November and their short term rental ended up being cheaper than what I originally paid with Japan-Wireless. I was able to go to their store the next morning and get everything set up and ready to go in about 30 minutes. Ended up canceling my order with Japan-Wireless and was refunded the amount.

I've also heard good things about Global Advanced Communications (http://www.globaladvancedcomm.com/pocketwifi.html) and might try them the next time.