Just ordered one wifi-to-go P-420 from them for 12 days, to be sent to our hotel in Osaka.
They claimed the battery life can go last whole day for most of the users but for continuous usage, should last 6 hours as they put in a larger battery. I won't count on it though. Going there to buy a rechargeable battery tank just in case. It comes with a USB charging cord (I think may come with a charger plug) so one can charge on the go. I hope 2 tanks should be able to support our phones and wifi device.
If you are interested to order/rent one, do it early as by looking at the deadline, one needs to order 2 weeks in advance.
@Jep: The WIFI-to-go is like a mini hotspot, the sim is inside the device as as long as your phone has wifi connection, you can use. And you can set password for it as well.
It can connect up to 5 devices. If you have that many devices, I suggest you do a rough calculation on how much data you are going to use, before deciding getting the unlimited or 1GB ones.