I've never owned the lens, but from what I've read the image quality is superb from that lens. In fact, they say there's only one lens that's marginally sharper--the 300mm f/4 L non-IS. ;-)


See what Bryan has to say:


Canon EF 300mm f/4.0 L USM Lens Review


Canon EF 300mm f/4.0 L IS USM Lens Review