Is the Sigma 10-20mm f/3.5 EX DC HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 14mm f/3.5 test image. Observe the contrast and resolution of these results.
Is the Sigma 10-20mm f/3.5 EX DC HSM Lens set to 14mm f/3.5 sharper than the Sigma 12-24mm f/4.5-5.6 EX DG HSM Lens set to 15mm f/5.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.