You are aware there are two versions of this lens - both versions are supposed to share the same optical quality but the more recent one called the "Sport: model is reported to have a much better build quality + Sigma is supposed to have improved their quality control.
The newer version also allows you to customize many features such as programming it for more accurate or for quicker focusing - but you need to buy the Sigma module to do the work,
The latest version is selling in the mid-#3500 range,
As soon as the holidays are over I'm going to order one with the adjustment module and 1.4 tele-converter
Here is a link to a review of the most recent "Sport" model - and as you're a Canon shooter you won't have the problem of the lens rotating opposite that of a Nikon,
http://photographylife.com/reviews/..._campaign=sigma-120-300mm-f2-8-dg-os-hsm
