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 125mm f/6.3 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 125mm f/6.3 sharper than the Canon EF-S 18-135mm f/3.5-5.6 IS Lens set to 24mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.