I have used it mostly in the US, but also have used it some in Mexico, Canada and some of the islands. I am a pilot for a fractional airline, so I live about 1/2 my life out of a suitcase.
I have used it at all the major chains of hotels, Hilton, Starwood, Holiday Inn, etc.
In addition to the airport express itself, I carry an ethernet cord, and both the longer wired cable (from an old macbook power supply?) and the "duckbill". Although I rarely need any of them, it sucks if either the internet cable is not provided, or the provided one won't reach enough to use the normal "duckbill" flip up US adapter.
I don't know about japan, but as long as you have the right plug I can't imagine it not working.
P.S. It's fantastic not toting a laptop around. I learned early on even the plastic 13" macbook weighs a ton after a week on the road.