If you are with Verizon for cell service - as we are - well, the sad news is their global plans kinda suck, especially for data. Their plans top out at 100 MB for a month's data allowance, which is about the equivalent of 3 Facebook posts. For this trip, I decided to go with their Travel Pass plan, which allows you to access your current plan allowances. You pay $10 for a 24-hour period, which starts as soon as you access cell service. Don't use it? Don't pay. So while you are essentially paying to access a plan you are already paying for, at least you can get at all that data you aren't using back home, for a relatively small fee.
And you may not even need it. I think I used it twice. The rest of the time, I was on free wifi (yes, security friends, I can hear you squealing right now). It's slow, and a bit of a pain (takes a while to show up, then sometimes you have to "log in" via browser, which usually consists of hitting a button "Use Free Wifi.") But it's everywhere - Starbucks (naturally), 7-11, rail and subway stations (but interestingly, NOT on the shinkansen), most stores, cafes, restaurants, and of course hotels. We had a couple of apps to help us find free wifi (Travel Japan and Japan Wi Fi) but honestly, mostly I just went to settings on my phone and scrolled through the list of available networks.
Related: your devices will charge - a little more slowly at 100V - without a converter or adapter. You only need an adapter if you are bringing a laptop or other appliance that requires a 3-prong plug.