I received a Sigma 15-30 several weeks ago and must say that I am well pleased. I did extensive research before buying the Sigma. It was a decision between the Canon 16-35L and the Sigma 15-30. The reviews I read generally rated the Sigma images to be at least as sharp, and sometimes sharper than the Canon. The extra 1mm is actually alot more noticeable than you would think. I paid just under $500.00 including shipping and the Canon is at least $1300.00... those are the pros....now for the cons...The colours are said to be slightly better, you get the speed of 2.8 across the entire focal range, the focusing is quieter and faster, the build is said to be more rugged, and you have the extra 5mm on the long end with the Canon. My particular needs, shooting style and budget added up to a pretty clear choice in favor of the Sigma. I do a great deal of prep sports and the Sigma was purchased for some specialty sports shots such as baseball teams in the dugout and some other odd and creative stuff, to include interiors and groups of people in limited spaces.
This is my 2 cents worth and I hope it helps. I almost failed to add that my main camera is a D60.