Ok, seriously... if you're referring to the distortion mentioned in (http://www.arthurdomagala.com/blog/2011/08/sigma-12-24mm-ii-lens-review/
) I would NOT WORRY ABOUT IT. I've NEVER seen anything close to the distortion documented in that review. I primarily use this lens for architecture -- like akclimber, abandoned buildings, bridges, city lines, etc -- and the lines stay almost perfectly straight. There is definitely distortion (elongated lines) on the edges, but the that is typical of ANY UW (even the mighty nikkor). It would be like complaining that a fisheye doesn't keep straight lines -- Pure baloney!
I'm also with akclimber on this, I typically don't use it for landscape as I don't like how the focal point seems too distant and the overall composure looks like a mess. Again, you'll get this with any UW as it's a composition thing.
Do I wish it was a f/2.8? Nope. Why? Because aside from the extra 1.5 stops gained, I would never utilize that kind of DoF. DoF is almost meaningless with an ultra wide. And knowing that the lens is much sharper once you get to f/8, I never use f/4.5... so why would I use f/2.8?
I also like this lens because it fits very nicely in my lineup. 12-24mm, 24-70mm, 70-200mm. Though again, like akclimber, I almost always utilize this lens between 12mm and 16mm. Any longer, and I'll just pop on my 24 since it's a 2.8 and my "go to" lens.
While I don't use the 12-24mm Sigma a lot, I always have it in my bag since there are plenty of occasions where I want to go REAL wide in a confined space. I do a lot of urban exploring, so it spends 90% of its time on a tripod stopped down to or past f/8. And given the nature of urban exploring (abandoned buildings, confined spaces, avoiding Detroit muggers) this lens fits that need perfectly.