Is the Canon EF-S 17-55mm f/2.8 IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 55mm f/2.8 test image. Observe the contrast and resolution of these results.
Is the Canon EF-S 17-55mm f/2.8 IS USM Lens set to 55mm f/2.8 sharper than the Sigma 17-50mm f/2.8 EX DC OS HSM Lens set to 35mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.