Great article on hiring great programmers, as opposed to good ones.
What's interesting is when Joel takes the stats of min vs max hours and then only looks at the top 25% (in terms of grades) - the differences in time are still there. As he notes with these studies: The quality of the work and the amount of time spent are simply uncorrelated.
It's not just a matter of "10 times more productive." It's that the "average productive" developer never hits the high notes that make great software.
Joel on Software - Hitting the High Notes