Is the Sigma 24-70mm f/2.8 EX DG Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 24mm f/4.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 24-70mm f/2.8 EX DG Lens set to 24mm f/4.0 sharper than the Canon EF-S 18-55mm f/3.5-5.6 IS Lens set to 18mm f/3.5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.