Is the Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 125mm f/6.3 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens set to 125mm f/6.3 sharper than the Sigma 18-200mm f/3.5-6.3 DC OS Lens set to 70mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.