From DXO Mark
The folks at DXOMark have tested the new Sigma 18-35 f/1.8 DC (APS-C) HSM lens and found to be extremely sharp. It turns out that it's the sharpest APS-C lens that they have ever tested. The performance rivals that of Canon's 24-70 f/2.8L II lens.
“Although it is unlikely a user would buy several full-frame wide-angle lenses specifically for an APS-C format camera, the fact remains that the Sigma 18-35mm f/1.8 DC HSM is an outstanding performer optically.
If we want to be picky, the Sigma lacks image stabilization and the zoom range isn’t as wide as some rivals, but at $799 the lens is remarkable….”