I've replaced my Tamron 70-200 f/2.8 with this Canon L lens and the difference between the two is monumental. After lugging this big guy around I will insist that this lens (whether it is the Nikon or Canon versoin) is a must-have for any photographer and should always be in your bag. It is incredibly versitile, it is built like a tank, has an ultra-fast USM motor for precise and consistant AF and the images are crisp and the colors are amazing.
Make no mistake... the Canon 70-200 f/2.8L (original, IS, or IS II) is that good and is ESSENTIAL to own.
Photo Courtesy of the-digital-picture.com