Is the Sigma 18-50mm f/2.8 EX DC Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 35mm f/3.2 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-50mm f/2.8 EX DC Lens set to 35mm f/3.2 sharper than the Tamron SP AF 17-50mm f/2.8 XR Di II VC LD IF 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.