there is a pretty big difference between good prospects and the #1 hitting prospect in baseball. there are lots of good prospects (lastings milledge, travis snyder, willingham, pence, etc. etc.. ) with those type of players you don't reall know when they are going to pan out, but can assume that eventually they will..
but as far as the #1 major-league hitting prospect goes (like wieters) there is a pretty proven track record..
off of the top of my head, i think its something like this.
2009 - Wieters
2008 - Longoria
2007 - Braun
2006 - Hanley
2005 - Ryan Howard
2004 - David Wright
2003 - angel Barroa
2002 - Eric Hinske
2001 - Pujols
somewhere in here is Ben Grieve, who obviously was a bust.. but only 3 busts in 10 years is good enough for me..
i think thats a pretty dominant track record. and if I had drafted every one of those players too early every year, I would have a pretty dominant keeper team right about now..