Is the Sigma 12-24mm f/4.5-5.6 EX DG HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 24mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 12-24mm f/4.5-5.6 EX DG HSM Lens set to 24mm f/8.0 sharper than the Canon EF 16-35mm f/2.8L II USM Lens set to 24mm f/11.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.