Is the Sigma 24-105mm f/4.0 DG OS HSM Art Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 24mm f/4.5 test image. Observe the contrast and resolution of these results.
Is the Sigma 24-105mm f/4.0 DG OS HSM Art Lens set to 24mm f/4.5 sharper than the Canon EF 24-105mm f/4L IS USM Lens set to 28mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.