From a list of 25 youngest draftees as of last summer:
Robert Swift, Lampe, Karolev, Bender failed to varying degrees and didn't have an impact. Bruno Sundov, second round pick. Amir Johnson - second round pick, hung around a long time, but not much impact (7p/5r).
Sekou Doumbouya, we'll see.
Bill Willoughby, Darryl Dawkins, and Bob Santini were long enough ago that it doesn't matter.
Ersan Ilyasova - second round pick, went back and forth to Europe. Milwaukee got their money's worth from their pick, though - We'd be in good shape if we got the value from 18 that Milwaukee got with that second round pick.
Superstars are the best case, obviously. Hall of Famers - Dwight stayed 8 years, McGrady 3 and LeBron 7 (all leaving for greener pastures). Kobe Bryant stayed forever. But, we can't draft Americans out of high school anymore. Today they would be older, better known, and wouldn't be there at 18.
Atlanta matched an offer sheet for Josh Smith's second contract, so that's the market value case I mentioned. Utah matched CJ Miles' offer sheet, then he left at age 25. Biedrins was resigned with competition, also market value.
Guys who went to a different team in six years or less:
Jermaine O'Neal, Darko

, Al Harrington, Dorell Wright, Andrew Bynum. (Bynum was a star drafted out of high school, but was essentially done by his second contract.)
Greek Freak is the lottery ticket I mentioned. Kirilenko was also a max player.