I originally picked this lens up because I wanted something a little wider than my 35L 1.4 and to cover those times I didn't want to carry all of my primes. I almost turned it down but for $200 I could not resist.
I used it for a wedding a few weeks after purchasing it. I was hesitant especially during the first dance (low light), but it focused fast and the images are sharp.
I've read a couple of reviews about contrast, but who cares when you're dealing with digital.....just bump up the constrast to your liking.
Anyway, this is a very nice lens and I would compare it favorably to Canon's 24-70 2.8L and Tamron's 28-75 2.8.
BTW, I did have the Tammy but I needed something wider than 28. The build quality of the Sigma is much better than the Tammy and is VERY close or even with the Canon.