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 35mm f/8.0 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 35mm f/8.0 sharper than the Canon EF-S 15-85mm f/3.5-5.6 IS USM Lens set to 50mm f/16.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.