Is the Canon EF 70-300mm f/4-5.6L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 100mm f/4.5 test image. Observe the contrast and resolution of these results.
Is the Canon EF 70-300mm f/4-5.6L IS USM Lens set to 100mm f/4.5 sharper than the Nikon 70-300mm f/4.5-5.6G AF-S VR Nikkor Lens set to 135mm f/5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.