I came across this interesting article in the Globe and Mail the other day :
globeandmail.com: Japan, my Neverland
I have tons of friends who went to Japan after university. They figured that it was their only chance to do so and figured they'd come back after a year or two of teaching English.
Of course that's easier said than done because Japan has a way of captivating you. Especially if you happen to be male.
Needless to say, many of them came back after 4 or 5 years only to realize that they'd forgotten a lot of the things they learned to get their comp sci or engineering degrees. They also went through a period of withdrawal as they experienced reverse culture shock. Some of them became a little depressed as they met with their former classmates who now had a good paying job, fancy car, large home, etc.
I also have friends who haven't been able to let go. They continue to live in Japan as their daily lives consist of 90min commutes, overtime work, tobacco, alcohol and tons of skanky women.
If you're really fascinated in Japan, perhaps you should consider a yearly vacation. If you're deadset on living in Japan, do it for the right reasons and make sure you have a plan in place... complete with an exit strategy.