I have the Canon adapter from when the M Series was first introduced. Vello introduced an adapter at the same time. I would suspect that the current adapter is still the same model introduced a few years ago. How well it works with newer bodies is an unknown, but it worked well the first and second generation of M Series bodies, AFAIK. I have heard hardly any complaints about it, which means little to nothing.
However, what lenses do you plan to use with the adapter. I quickly found out that only the smallest EF lenses are worth the trouble of using with M Series bodies because of their ultra compact size and short battery life. The EF/EF-S pancake lenses are perfect.
The consumer oriented primes are small enough to work fairly well. A lens like the EF-S 18-135 would be unbalanced towards th lens end of the combo, plus there is the battery drain issue. Carry at least two spares.
I have used the Rokinon 14mm T3.1 with the M3, and have been able to get off hundreds of shots in one day in eco-mode. The lens dwarfed the camera body. But, once it was dialed in to the hyperfocal distance using focus peaking, I could shoot all day without having to worrry about focus.
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