Here are the numbers from 82games about draft picks landing "star players":
http://www.82games.com/nbadraftpicks.htm
1st pick: 70%
2nd pick: 60%
3rd pick: 85%
4th pick: 60%
5th pick: 60%
6th pick: 25%
7th pick: 30%
8th Pick: 35%
And even among the picks that don't become "star players," a large percentage become solid players, starters on playoff teams and 6th men of the year, etc.