Funny how a simple question can sometimes trigger a discussion like this..
I love EOS M a lot (have three of them), but to me its strength is small size. Adding an adapter (which has the same thickness) and then attaching a large EF lens to it defies the purpose because it makes the final setup large again. I would rather use 70D or something because the length would be the same and the width just a bit more. I considered Viltrox EF-EOS M2 adapter recently, but after watching this review (
I decided against it - it degrades image too much. Can't stand the softness overall and especially in the corners (at large apertures) and ugly violet CA around high contrast edges (see 12:44 in the video where he shows some trees against bright sky). Some people are less sensitive to these things, I guess.
Metabones recently released their own focal reducer for the M mount, but at $480US it's not exactly a bargain, though I expect it to be much better optically.
To me personally, if larger apertures are needed, EOS M should be used with those 7artisans/Meike/Kamlan primes, which are relatively inexpensive and keep the final setup small. I'm planning to get Kamlan 28mm F1.4 or Meike MK-35mm F/1.4 because they have good reviews.
Overall, as it stands atm, we don't have a perfect camera for ML, each has its own strengths and weaknesses. 5Dm3 is the best overall, but it doesn't have DP auto focus. 70D has it, but can't do 3x3 binning (as the rest). EOS M is small but its focusing system isn't the best. This is why I personally have different cameras and use them differently.
I would never use EOS M as the main camera to film a paid job. It just looks silly, like if you are using a compact or something

. But it's great for run and gun situations.