The previous version of Sigma 70-200 lacked of a very important feature: image stabilization. Now, Sigma has created a 70-200 that has both stabilization and fast autofocus, together with the promise of first rate image quality and a relatively high price tag, that puts it in direct competition with the Canon 70-200 II. How does the two lenses compare? Is the Sigma worth $ 1700? Let's see :-)
As usual, I recommend to give a look to the page Testing and reviewing a lens: it will help you to understand better this review and my testing methodology!
 
Specifications (compared with similar lenses)