Over the past 6 months, I have visited Japan on several occassions, and as driving was required the issue of navigating ones way around Japan looked like a potentially frightening experience.
After web searching, I found a site which offered GPS maps for whole of Japan. Unfortunately or fortunately the maps are only available for Garmin range of units (for last 7 years I have preferred Navman units and to a point still do), so I invested in a nuvi 1350T and ordered the maps from the above mentioned site.
The installation, of the maps, was very simple and I have been quite surprised at the quality and accuracy of using the system/maps, and have found it certainly has made life very easy in the navigating and driving. The only issue is the entering of addresses etc, as Japan does not in general work on a street names and numbering system. To solve this issue, I use both Google and Google Earth to find the longtitude and latitude and enter these points. It sounds a long winded method, but a few moments in preparation is well worth the effort.
For anyone contemplating Japan as a test for GPS and driving in a real foreign language region, I certainly suggest these maps in English.