Is the Sigma 18-200mm f/3.5-6.3 DC Macro OS HSM C Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 18mm f/3.5 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-200mm f/3.5-6.3 DC Macro OS HSM C Lens set to 18mm f/3.5 sharper than the Tamron 18-200mm f/3.5-6.3 XR Di II LD Macro Lens set to 28mm f/4.5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.