Is the Canon EF 70-200mm f/2.8L USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 200mm f/3.2 test image. Observe the contrast and resolution of these results.
Is the Canon EF 70-200mm f/2.8L USM Lens set to 200mm f/3.2 sharper than the Sigma 70-200mm f/2.8 EX DG HSM Lens set to 280mm f/11.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.