Is the Sigma 17-70mm f/2.8-4 DC Macro OS Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 28mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Sigma 17-70mm f/2.8-4 DC Macro OS Lens set to 28mm f/5.6 sharper than the Canon EF-S 17-85mm f/4-5.6 IS USM Lens set to 50mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.