For those travelling to Japan and thinking of using a prepaid SIM card, a few things worth noting:
1. I've tried B-Mobile, it's pretty slow. Softbank's one is decent as is UMobile, available from 7-Eleven.
2. It might be worth getting a Voice/Data SIM rather than a data only SIM. Why? Let's say you're using Line or WhatsApp or anything that might require sending you an SMS for authentication purposes. You will not be able to receive those SMSes with a data SIM ... whereas a voice SIM will work even if you're out of credit.
3. Watch out for the bands your phone is enabled for. Make sure they match up with at least a few of the bands used by whoever you're planning on getting a SIM from.