ESPN has a long profile of Japanese baseball and Yu Darvish. I recommend the video too.
ESPN.com - E-ticket: Dice-K 2.0
Whether the wildly popular Darvish remains Japan's best pitcher until he is eligible for free agency in five years or is posted earlier (and perhaps much earlier) for a major league team to sign depends on what makes the most financial sense for the Nippon Ham Group, and not necessarily what makes the most competitive sense for the Fighters.