I had that lens' predecessor and I shoot a 40D. Overall, the lens was OK but had a great deal of distortion at 24mm. Flare was generally well controlled but could be a problem with lights or bright objects in the frame. My copy had front-focusing issues which is why I eventually sent it back. Nearly all the issues I mentioned have supposedly been fixed in the new lens. But the new lens also now costs $900 instead of $550. By all accounts Sigma continues to have spotty quality. They'll make a masterpiece and then the next lens down the line works as if assembled with a hammer, chisel and blow torch.

Don't know if you've considered it but $900 puts your into the Canon EF-s 17-55 f/2.8 IS USM. Obviously still an f/2.8 but you get IS and don['t have to give up so much on the wide-end. Of course, you still give up some on the long-end. I switched to this lens a few months ago and love it. Did I mention it has f/2.8 AND image stabilization?