Please learn to use your friendly neighborhood spider-Google next time. This subject has been beaten to death. Some here:

http://news.ycombinator.com/item?id=615846

http://news.ycombinator.com/item?id=1175223

http://news.ycombinator.com/item?id=1578380

http://news.ycombinator.com/item?id=1787241

http://news.ycombinator.com/item?id=931832

http://news.ycombinator.com/item?id=1758554

http://news.ycombinator.com/item?id=1840197

http://news.ycombinator.com/item?id=1756924

The search literally took maybe 3 seconds tops...