I'm a wildlife shooter and recently invested in Canon's new 600mm mark III. It's light, incredible and works well with my 1DX II and 5D Mark IV. That said, I'm worried how this lens, along with the rest of my EF lineup, will interact with the new mirrorless bodies. I don't have the EOS R as it was too slow for my needs, so I'm keeping my fingers crossed that the adapter will not slow down autofocus one bit....it does add more connection points regardless. Even a very minor or small lag can be a deal breaker in the wildlife space. I'll want to buy this new pro mirrorless body in a few years as it will be their fastest camera and an EVF along with a silent shutter are all major positives as far as I'm concerned. That said, the major drawback will be any loss of capability when using the adapter with the EF telephoto line. I can't imagine Canon would be releasing a full range of super telephoto RF glass by 2021. And if they do pull something like that off, they should've went toward that entire direction earlier rather than releasing recent EF updates to the 600mm and 400mm f/2.8 lenses...as well as an upcoming 1DX Mark III.