Is the Sigma 120-300mm f/2.8 EX DG OS HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 250mm f/2.8 test image. Observe the contrast and resolution of these results.
Is the Sigma 120-300mm f/2.8 EX DG OS HSM Lens set to 250mm f/2.8 sharper than the Nikon 70-200mm f/2.8G AF-S VR Nikkor Lens set to 280mm f/4.5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.