Is the Sigma 70-200mm f/2.8 EX DG HSM II Macro Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 280mm f/4.5 test image. Observe the contrast and resolution of these results.
Is the Sigma 70-200mm f/2.8 EX DG HSM II Macro Lens set to 280mm f/4.5 sharper than the Canon EF 70-200mm f/4L IS USM Lens set to 135mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.