It looks like an interesting lens. I would have to read some IQ comparisons before making a decision.
The Canon AF dominance makes a lot less difference on wide lenses, so if the IQ is as good or better, I would consider 3'rd party. The VR and the wide aperture would make a difference in hand held shots.
I was trying to shoot some hand held pictures with my 17 the other day around sunset. Let's just say I'm not posting any of those. (A few actually were sharp, but I just didn't like them)
With a tripod, I'd never use the F2.8.
I've also been wanting the 11-24, so I am going to research the Sigma 12-24 as a lower cost alternative. Wider is better.