Is the Sigma 18-200mm f/3.5-6.3 DC OS Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 200mm f/7.1 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-200mm f/3.5-6.3 DC OS Lens set to 200mm f/7.1 sharper than the Sigma 18-250mm f/3.5-6.3 DC OS HSM IF 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.