As a relatively new owner of the Sigma, I have yet to fully get to grips with its detailed character.
For me the zoom and price difference alone were significant reasons to choose it, providing the image quality was decent. Used on a 7D, I find it visibly softer at f/2.8 than when stopped down at all so for critical sharpness that may be a consideration. It's good enough, but if you need the absolute best wide open, you may want more than good enough. I haven't noticed corner issues on crop sensor. AF speed is ok, but not super fast.