I just got back from Japan and here are my 2 cents...or Yen:
1. When I got to Narita, I made a call out on my Droid to a guy in Japan no problem...but then things started to get weird.
2. When I turned on my phone at the airport, I got a welcome text then a notice that I had over $100 in data charges. I then turned off my phone. Then I bought the international data package before you leave (Via Skype on my laptop to tech support).
3. Not all areas are covered in Japan: I was in Mitaka (Tokyo) and Tokorozawa (Tokyo) and my phone did not work. BUT other areas the data worked, but no cell.
Bottom line: buy the data package before you leave, don't expect the phone to work in all areas. For the most part, worked okay. I still rented a phone for my local calls anyway.