Canon Explorer of Light Danny Green has put the EF 100-400 f/4.5-5.6L IS II through the paces for the CPN. While there's no way the article would say anything negative about the lens, all the positives about it are worth noting.
“This EF100-400mm f/4.5-5.6L IS II USM lens has the new, special Air Sphere Coating (ASC) technology that incorporate what Canon calls a low refractive index layer. It took me a while to get my head around this, but it’s fascinating. Basically, when light enters a lens, it slows down as it passes from the air through the glass. This slowing down in speed is what causes reflections and flare, causing problems such as ghosting and contrast degradation.”
“It works with an EF1.4x converter* too, unlike the old 100-400 which would only let you manual focus with it fitted. With this new lens you can use AF no problem with the central focusing point. Another big step forward.”