Hi...
You can check out the reviews here and here.
The Sigma or *Bigma* is considered a contender but I doubt that it can beat the Canon 100-400L however; for the price and the 10x zoom factor it does well if without it shortcomings like it's vignetting and AF issues at the long end.
Please note that I recall a Sigma Service Bulletin whereby Sigma had issues with the AF system on some of there telephoto lenses. I'm not sure if that was addressed - one would assume it was.
On Flikr, there is a user that goes by the name *Airforcehound* or something and his go to lens was this lens until a recent mishap. You want to search there, the photos look great!
I personally don't have one, but I was curious about some of Sigma's telephotos including the 150-500.