Is the Sigma 24-70mm f/2.8 EX DG HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 50mm f/11.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 24-70mm f/2.8 EX DG HSM Lens set to 50mm f/11.0 sharper than the Nikon 24-70mm f/2.8E AF-S VR Nikkor Lens set to 50mm f/11.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.