Darvish has pitched against MLB hitters. Unlike some of the other Japanese pitchers who have come over here, Darvish is very big... 6-5/230 or something like that. He doesn't lack experience and he is far from being an unknown.
No high school pitcher has ever received $8 million to sign. I'm not sure what Darvish is worth, but if there were enough 8 million dollar high school kids (which there isn't) I would take 12 of those over one Darvish or any other free agent. 8 million is first pick of the draft type money.
Using that theory, I could have one Darvish or instead, for a lot less money, I could have Josh Hamilton, Adrian Gonzalez, Joe Mauer, Delmon Young, Justin Upton, Luke Hochever, David Price, Tim Beckham, Stephen Strasburg, Bryce Harper, Gerritt Cole. Or for that matter name any 12 MLB players you want, because all of them signed for less than 8 Million signing bonus.
What would you do? Then again, one single team never gets a chance at all those draft picks.