If you visit Japan the next time and you want to get perfectly good Internet access while there on your mobile phone I can recommend the b-mobile offer. On my last stay in Japan (May 2014) I tried their service for the first time and I was not let down.
They give you two options: The 1 GB prepaid option gives you 1 GB as fast as possible. The 14days prepaid gives you 14 days of limited speed coverage (300kbps).
I went for the 14 days prepaid option knowing that I might get some usage depending on where I go. The 300kbps where faster than I thought – at no given time I experienced any speed problems. The coverage was awesome since it just dialed into NTT Docoms 3G/LTE network.
For just under € 30 this is an awesome option for any traveller. Even better: You can pick it up at the Airport or you can have it delivered to your hotel! We tried both and it worked both as expected. Fast delivery, perfect service!