I have the 12-24mm II which I use with a 5DII. I also have a 17-40 so I can offer a direct comparison.
My Sigma copy is sharper across the frame than the 17-40 at equivalent focal lengths and apertures. The Sigma's extreme corners soften at its widest focal lengths. By 'extreme corners', I mean the outermost 3% or so of the image. This image from photozone.de illustrates it.http://photozone.smugmug.com/photos/1449271793_tsLbq28-O.jpg
This is one of those lenses that shows its best in real-life images rather than the lab. Where in the any of the following images on this page is corner sharpness an issue?http://www.ryanbrenizer.com/2011/11/review-sigma-12-24mm-mark-ii/
The Sigma's CA is low across most of the frame and reasonably low in the corners. I suspect this is the biggest advance over the MkI. I have to say that I've never had a problem with flare but at 12mm, it's sometimes a challenge to keep flare sources out of the frame.
Build quality is excellent and it has a nice chunky, heavy feeling to it. For what it's worth, the front element is a work of art.
Two downsides: first, the lens accepts rear gel filters so a polariser can't be used. Second, low light autofocus is noticeably less decisive than any of my canon lenses. On the subject of focus, for landscapes I usually set focus manually at the hyperfocal distance.
12mm is so wide that I treat the Sigma as a 'specialist' creative lens, for use in limited, specific circumstances. In general photography , I don't need to go wider than that offered by my 24-105 but when the right situation presents itself, the Sigma offers a truly unique and frequently stunning perspective.