DxOMark has tested and published the results on the Canon EF 500mm f/4L IS II and EF 600mm f/4L IS II lenses and found them to be very good. Although the EF 300mm f/2.8L IS II and EF 400mm f/2.8L IS II are 'sharper' on paper, suffice to say the Series II lenses are great but the burning question for the current Series I owners - is the incremental improvement worth the expensive upgrade? You can read the DxO article on their website.
For anyone thinking of owning the latest Super Telephoto lens and has the money, it is a no-brainer but for those with limited funds, a well maintained Series I version is a tempting alternative. I am waiting for my EF 200-400mm f/4L IS USM Extender 1.4x lens to arrive. Although a very expensive lens, its unique zoom and built-in extender makes it worthwhile for me. I will put it through a series of wildlife photo shoots as soon as possible and write a review on it. In the meantime, you can read my preview of the lens here.