The Sigma 17-50 / 2.8 EX DC OS lens is a durchnittliches, unfortunately it has some weaknesses outside the image-center, but you do not notice this as the average Joe photographer really.
At 17mm, and shortly before the 50mm it reduces slightly. The aperture 2.8 is not as sharp as the 4 or 5.6. But this fählt on only during testing. The resolution, to which it indeed matters is average on a EOS 60D, since the edge of the picture builds this a lot from.
However, you have to say, there is no L Canon lens and thus it must have these limitations.
I love to take pictures with it, if only when I think of the purchase price.
It's definitely better than the processed Tamron SP AF 17-50mm Di II VC lens 2.8.
The autofocus is due to HSM engine also afloat. The Tamron is as very slow.
The accuracy of the AF is super, at least in my constellation EOS60D and Sigma EX.
The sharpest and best results get with 25mm aperture and 4 or 5.6.
Aperture 2.8 you should only take if there is too little light.