Fairview Mall in Toronto.
Smaller version on flickr.

I've been using the Sigma 12-24 lens for a couple of weeks now and so far I’m impressed by how sharp this lens is edge to edge. Even at larger apertures it provides good images. The downsides are more than usual glare and the fact that you can't use any filters at widest end, but both are completely understandable due to the super wide nature of the lens.