While in Japan, I decided to rent an iPhone from Global Advanced Communications. I had no issues with this service, and it was a pleasure to have a cell phone that I knew how to use. I picked it up from the post office in Narita upon my arrival. As soon as I handed them a printout of my e-mail from GAC, they knew what I wanted.
It was only after a week or two that I realized that I had upgraded to iOS 7 in violation of their instructions. I phoned them and discussed this issue, and they said that as long as I didn’t turn on a passcode or “located my iPhone”, it was OK.
I didn’t try to connect to iCLoud. I had some issues with having the phone play nicely with one of my iMAP e-mail accounts. For Gmail, the setup was initially refused by the server, but I logged in via Safari. After I was asked for a secondary password, everything was OK. I got messages on my linked e-mails to the effect that someone had logged into Gmail from a new iPhone.
One larger issue is that there is a distinct lack of public WiFi in Japan. If you are just in central Tokyo, you could probably get away with what you can find (for example the free NTT service for which you register at Narita) but for the rest of Japan this is a real issue.
I ended up renting a portable router from this outfit, through a Japanese contact.
It was about 7000 yen for three weeks. Now I see that GAC also rents the same unit for just a bit more money. The router was LTE and so it was much faster than the 3G iPhone 4s. Note also that GAC’s phone are linked with Softback, and so it does not allow data tethering.
One other note: for the rental service that I was using, there was a soft data cap: above a certain amount of data in a 24 hour period, they would start throttling in the evening. The cap was only ever exceeded when my daughters watched more than a couple of hours of streamed video during the day. I see that GAC also documents a data limit.