First of all, its DxOMark score is pretty impressive for a telephoto zoom:
Mounted on a 5D Mark II the results are superior or close to 10 (DxOMark Score) on the whole focal range at 5.6.
This means that the global image quality will be high for every focal used, even when lighting conditions are difficult.
The resolution is very homogenous in the field, however it is not very high.
Canon EF70-300mm f/4-5.6L IS USM vs Canon EF70-300mm f/4-5.6 IS USM
The full comparison is available here : Canon EF70-300mm f/4-5.6L IS USM vs Canon EF70-300mm f/4-5.6 IS USM
Every aspect of image quality measured on dxomark.com shows improvements.
For this focal range, Canon provides here a very good L-series lens at an affordable price.
Here are other interesting camera lenses comparisons:
Canon EF70-300mm f/4-5.6L IS USM vs Canon EF 75-300mm f/4-5.6 IS USM (mounted on a Canon EOS 5D Mark II) (please, remember to change the camera body on the drop-down menu).