I went to Japan last week. I was there for two weeks. I hired a wifi device from Global Advanced Communications. It cost me around 9,000 yen for two weeks. Delivered to my hotel on arrival. I travelled from Tokyo to Hiroshima, then Takayama then back to Tokyo before heading north to Fukushima. It worked most places. Some of the freeways in the hire car it didn't pick up signal but I had an English GPS.
It was amazing. A real lifesaver. I could Skype, Facebook, email, iMessage text and YouTube. Google maps worked very well and so did apple maps.
If you need any info just send me an email.