Is the Sigma 120-300mm f/2.8 EX DG HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 300mm f/3.2 test image. Observe the contrast and resolution of these results.
Is the Sigma 120-300mm f/2.8 EX DG HSM Lens set to 300mm f/3.2 sharper than the Canon EF 100-400mm f/4.5-5.6L IS USM Lens set to 350mm f/6.3? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.