Hey,
I have a Canon 70-200 F2.8 non IS...its a brilliant lens and great color and clarity.
But that being said, the Sigma in the same range is much cheaper, and my dad recently bought one. I compared the images on my camera with both lenses and i couldn't tell the difference, aprt from my dad saved over 400 dollars canadian...that can buy you another lens..
go into the store with your camera and try it..take you camera back home and see what the pics look like..you wont' be disappointed with either..
I believe that Sigma has just updated the lens to be faster, quieter and an even better image...
thegary