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 28mm f/5.6 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 28mm f/5.6 sharper than the Canon EF-S 17-85mm f/4-5.6 IS USM Lens set to 35mm f/16.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.