Normally, "huge in Japan" is considered somewhat of a backhanded compliment. If you don't think so, try telling a successful artist, writer or musician that you have heard that he is very popular..in Japan.
"Popular in Japan" is a dubious distinction most of the time. It's like having the record for home runs in the minor leagues, or being the best and most popular basketball player in Turkey.
But according to this article on CNet, http://news.cnet.com/8301-17939_109-10108796-2.html, Japan is the only thing keeping Yahoo relevant. Believe it or not, Yahoo Japan is bigger than Yahoo in America. In practically every other country where Google and Yahoo do business, Yahoo seems to be holding a pretty steady lead on search engine usage over Google.
Rest assured, Google, like always, is making a comeback. But this serves as another example as to how you can't predict what people or demographics are going to do. Hedging your bets by having good placement on all the search engines is the best way to go about practical search engine optimization.