Most popular GPS devices in Asia?

I was in Japan for a recent trip and noticed that most of the taxis had GPS devices, but they were mostly brands I didn't recognize (not to mention many were in Japanese).

Any idea how much coverage Garmin, TomTom, Magellan etc have in Asia? I noticed Garmin doesn't have hardly any Asia maps for sale.

I travel to Asia quite frequently and would like to use my Garmin more on my trips.