To use their SIM cards, you need a SIM-free/SIM-unlocked device that supports W-CDMA/HSDPA/HSUPA. There is a list of compatible phones on their website, here.
It seems really easy to recharge, and might be a cheaper option for some students than to rent a pre-paid cellphone, especially if you won’t really be making calls.
For those students who’d prefer to have a phone with call capabilities, though, we have been recommending providers G-Call to our students, so far with great success! You can find more info on this service here.
As a last resort, there are still a few public phone boxes dotted around Japan, some with international calling capabilities!