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/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 55mm f/5.6 sharper than the Canon EF 17-40mm f/4L USM Lens set to 40mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.