I recently upgraded my telezoom from a 70-300 IS to a 100-400 L IS. Both of these lenses receive varied comments on the internet, with varying claims as to softness and sharpness. I was delighted with the extra sharpness and contrast I saw with the 100-400, not to mention the extra reach.

All shots taken on tripod with IS off, at maximum aperture. Pictures are 100% crops from the image centre, converted from RAW without any sharpening.