Is the Sigma 17-50mm f/2.8 EX DC OS HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 21mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Sigma 17-50mm f/2.8 EX DC OS HSM Lens set to 21mm f/5.6 sharper than the Nikon 17-55mm f/2.8G AF-S DX Nikkor Lens set to 55mm f/11.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.