Here is an example of a recent Micro Adjustment I did on a 5d Mark II with a Canon EF 24-70mm f/2.8L I USM lens.
The Auto Focus was off by 10mm (front focusing) and if you are shooting wide open that could be the difference of the bridge of your nose being in focus versus your eyes.
*Update*
I used a Datacolor SpyderLensCal Autofocus Calibration Aid. You can find them at BHPhotoVideo: http://www.bhphotovideo.com/bnh/controller/home?O=&sku=736849&is=REG&Q=&A=details
