I hate to be the jackhole that tells you to search, but there seriously have been a ton of threads on this, and here are 4 of them:

http://bikeforums.net/showthread.php?t=207346

http://bikeforums.net/showthread.php?t=233978

http://bikeforums.net/showthread.php?t=184502

http://bikeforums.net/showthread.php?t=84166

etc. ad nauseam