Is the Sigma 18-250mm f/3.5-6.3 DC OS HSM IF Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 35mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-250mm f/3.5-6.3 DC OS HSM IF Lens set to 35mm f/5.6 sharper than the Canon EF-S 18-55mm f/3.5-5.6 IS Lens set to 24mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.