I also own this lens, and tested it side by side against the Canon 70-200 2.8 L in studio, under controlled conditions. Myself and a coworker could not distinguish between the two, they were that close. They do have minor differences, such as one is slightly warmer than the other (don't remember which was which). As I needed a 2.8 lens and plan to use it there, I tested at that aperture as well as closed down, and again both the Canon and Sigma lenses were pretty much equal to each other.
I also compared focus speed, noise, and hunt between the two. Again, they were pretty much equal, though the Sigma is a bit louder. Of course, since the Canon lenses are pretty much silent, any sound is louder compared to Canon's USM. In a moderately lit office, they both hunted about the same, and both found focus about the same. Performance, build, and optically, they really are just about equals. The one performance difference is that the Canon focus down to 5 feet, the Sigma only to 6 feet.
The big differences are in function. The Canon lens zooms from right to left, while the Sigma zooms from left to right (a la Nikon). One difference that I think is slightly in favor of the Sigma is the zoom ring / focus ring layout. On the Canon, the zoom ring is fairly large, while the focus ring is a bit smaller. On the Sigma, the zoom ring is plenty big, while the focus ring is not only bigger, but it also sits closer to the zoom ring, making it easier / quicker to use, and if you are somewhat coordinated you could actually turn both at the same time with just one hand. The rings on the Canon are a bit more spread out, making that a little more difficult. In the end, probably not really that big of a difference. And finally, on the Sigma, the knob for loosening / tightening the tripod colar sits 90* away from the foot itself, where as on the Canon the knob and foot are side by side. This means instead of one protrusion (the foot) banging your knuckles or taking up room in your bag, on the Sigma, you have two.
But then, it is $500+ less...