One word of advice if you are planning to buy a SIM card for use in Japan -- buy it at the airport when you land!! It was surprisingly difficult to find a tourist-friendly SIM card to buy elsewhere in the country (but we finally found a place - see below).
We knew before we arrived that Japan wasn't the easiest place for tourists to buy SIM cards. The laws here only allow residents of Japan to buy SIM cards including voice and SMS, so tourists can only buy SIMs with data use only (which was fine for our needs).
We researched a number of companies that sold prepaid tourist SIMs at the airport, including at least one through a vending machine. But since our flight landed a few hours late, we were in a race to catch the last JR train for the night headed for Shin-Osaka (where we had a hotel booked). And we had two higher priorities than the buying our SIM cards: arranging for our luggage to be shipped to our hotel (only a fool in Japan brings big luggage on the train - much easier to courier it overnight from the airport!) and converting our JR Rail Pass vouchers into actual train passes. We got these things done with mere minutes to spare to catch that last train for Osaka. (Just to note, there WERE a few later trains, but our rail passes couldn't be used on them).
We figured that we would be able to find SIMs in town in Hiroshima (where we went after our quick hotel stop near Shin-Osaka), but none of the phone company stores carried them. While many of the companies did offer them, they ONLY sold them at their Airport kiosks.
Thankfully we did discover that Bic Camera sold a SIM that we could use, and there are BIC stores all over Japan. Their SIM offers 2GB of data to use over 3 months and costs 3780 Yen (just over $40 Canadian).
I would have preferred the plans that some of the other companies were offering - unlimited data for a 14 day period for about the same price (with the caveat that if you use more than 200Mb per day the speed slows down until midnight). I'm a data whore so even with using wifi where possible, I will probably burn through my 2GB before we leave and have to pick up another card.
One tricky thing about the card from BIC is that it does not automatically change your APN settings. On an Android device this is apparently easy to change, but iPhones don't make it easy (as most people know I'm an Apple fanboy, but this is one time where I wish Apple would open things up a bit -- some of the early versions of iOS allowed one to manually change the APN settings, but they locked it down a few years ago).
There are a few websites that allow you to create a configuration profile to change the APN, but we couldn't seem to create a profile that worked. Thankfully with a bit of Googling I found this link to a profile that did the trick (warning that if you are on an iOS device clicking this link will attempt to create a configuration profile for you - but no harm done if you just hit cancel)
And yes, I realize we should be enjoying the sites and sounds of Japan rather than having our faces staring at our iPhones half the time. But this is just how we roll.