…. and they're pretty good.
DXO Mark has tested the Canon EF 500 f/4L IS II and the EF 600 f/4L IS II and found them to be good. :) The 600 appears to be a tad bit sharper than the 500, which is a reversal from the previous version of the lenses. As has always been, these two lenses are not as sharp as the EF 300 f/2.8L IS II or the EF 400 f/2.8L IS II.
Image quality isn't the most important thing in regards to these lenses. The remarkable job that Canon has done improving usability cannot go unnoticed. The new 600 and 400 are shining examples of the best of what Canon has to offer and they're unmatched in the industry (I've used both the Nikon versions and they don't compare in this regard).
“Both of these lenses offer exceptional performance that far exceeds anything else available in a Canon mount. They are also built to last, with fantastic build quality. If you’re looking for the best, these should be at the top of your list. The only things to keep in mind is that they are both heavy lenses – though much lighter than their predecessors and not unduly heavy for a lens of this focal length and aperture – and they are also expensive, but the best usually is.”