Is the Sigma 17-70mm f/2.8-4 DC Macro OS Contemporary Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 17mm f/3.2 test image. Observe the contrast and resolution of these results.
Is the Sigma 17-70mm f/2.8-4 DC Macro OS Contemporary Lens set to 17mm f/3.2 sharper than the Canon EF-S 15-85mm f/3.5-5.6 IS USM Lens set to 15mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.