Is the Sigma 18-250mm f/3.5-6.3 DC Macro OS HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 50mm f/5.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-250mm f/3.5-6.3 DC Macro OS HSM Lens set to 50mm f/5.0 sharper than the Canon EF 100mm f/2.8L IS USM Macro Lens set to 100mm f/2.8? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.