Is the Sigma 120-300mm f/2.8 EX DG OS HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 180mm f/2.8 test image. Observe the contrast and resolution of these results.
Is the Sigma 120-300mm f/2.8 EX DG OS HSM Lens set to 180mm f/2.8 sharper than the Canon EF 70-200mm f/2.8L IS II USM 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.