I want to point out this interesting piece of historical draft research from Nate Gilmore over at Puckett's Pond. Gilmore looks at high school hitters, high school pitchers, college hitters, and college pitchers, drafted in the first round from 1995 through 2008. He looks at both how often first-round picks in each category reach the majors and how successful they are once they get there.
Of particular interest, he concludes that college hitters are generally the safest investment, both in terms of their chance to reach the majors and their chance to thrive and become regulars or All-Stars once there. High school hitters are much riskier and don't become strong contributors as often as the college players.
Of the moundsmen, high school pitchers are far riskier than their college counterparts, with a large chance (about 45%) that they will fail completely and not reach the majors at all. However, the high school pitchers who do survive Darwinian selection in the minors and reach the majors, become stars more often than college pitchers. Drafting high school pitching is very much a high risk/high reward endeavor.
Gilmore also looks at how long it took players from each category to reach the majors. Take a look at his study, it is very interesting.