I still haven't tried it since I have internet access at the conference and at the hotel.
The thingy that you see here is however a pocket wifi. You can't just buy a pre paid sim card and insert it into your phone while in Japan.
You either rent a phone or a pocket wifi.
The pocket wifi is basically a gadget that connects to the 3g network, and then shares the internet connection via wifi. Will try it tomorrow when I'm out walking.Update: Just tested the pocket wifi between two sessions, and it worked!