I'm in the midst of reviewing the new Sony A7r II camera. One of the most pleasant surprises so far? Just how fast autofocus can be with Canon lenses and a Metabones adaptor. Watch this video, and see what an amazing improvement this is from anything that came before.
Just like with modern smartphones, investing in a camera typically means being locked into a specific brand's ecosystem. With cameras, that means glass. Lenses are expensive and designed to work only with their own brand: adapting a lens from one brand to a camera from another typically means sacrificing key features like autofocus. But as you can see, the new Sony A7r Mark II makes huge inroads into solving this dilemma.
This means that if you want what the A7r II has to offer -- great stabilisation, high res photos -- but are heavily invested in Canon glass, you can look forward to using those lenses without fear. The adaptor I used is from Metabones, and costs upward of $US400 depending on the model you get. That's a chunk of change, sure. But it affords you a whole hell of a lot in terms of expanding the possibilities of your new camera, and it's definitely cheaper than buying Sony lenses to replace your existing ones. (Results may vary with other, cheaper adapters.)
I've tested the A7r II with three Canon lenses so far: the 24-105mm f/4 L, the 35mm f2 IS, and the 16-35mm f4 IS. All three have worked very well. There are certainly lenses that will do worse, particularly older Canon lenses. It's also true that advanced focus features like subject tracking will definitely see a big dip in performance -- meaning that for demanding action scenarios this might not be the dream setup after all. But I bet the vast majority of mirrorless camera users will be pretty happy.
I'm also an owner of the original A7, and adapting Canon lenses to that camera made for incredibly slow and unreliable autofocus. The new setup is infinitely better.
Sadly, the fast focusing does not apply to video. In my tests, engaging autofocus in video mode with Canon lenses produced a great deal of hunting, sometimes never landing on proper focus at all. Honestly though, autofocus isn't the preferred method for most videographers, so it's not a huge loss. Unfortunately for owners of Nikon lenses, there aren't any adapters that preserve the autofocus function on Sony bodies. You're out of luck. But for owners of Canon glass, this new capability is a wonder.