Is the Nikon 17-55mm f/2.8G AF-S DX Nikkor Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 24mm f/2.8 test image. Observe the contrast and resolution of these results.
Is the Nikon 17-55mm f/2.8G AF-S DX Nikkor Lens set to 24mm f/2.8 sharper than the Sigma 17-50mm f/2.8 EX DC OS HSM Lens set to 28mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.