Bobatkins.com tested and reviewed the Canon EF 70-300mm f/4-5.6 IS USM and wrote:
“..lens is sharper at f5.6 than the old lens was at f8. Don’t get carried away though, since despite using a UD glass element, it’s still not as sharp as the 300/4L. Even the new lens benefits by stopping down to f8. With an EOS 20D and at f8, on real world targets, image quality almost approached that of the 300/4L is some shots. However on resolution test charts the 300/4L showed higher contrast and resolution. Still, the performance of the new lens is pretty good.
..the new EF70-300/4-5.6 IS fits the bill as an ideal “walking around” lens for nature, travel and sports photographers. It’s small enough and light enough to carry around as part of a kit (it would pair well with an EF-S 17-85/3.5-5.6 USM) and the image quality and stabilization are improved enough over the old lens to justify its additional cost. ..”