Is the Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 70mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens set to 70mm f/5.6 sharper than the Canon EF 17-40mm f/4L USM Lens set to 17mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.