I have both the EOS-M and the 6D. The EOS-M is a great camera for specific tasks. I love having a small pocketable camera that I can take with me when I'm out with my family. I love using the image stabilized 18-55mm lens for shooting video, and I love the idea of having a back-up camera that takes fantastic pictures and can use all my existing lenses with the adapter. If I want to extend the reach of my long lenses, it's cheaper than the 1.4x extender and gives me 1.6x the reach of my FF lenses, and it works with ALL my lenses, unlike the 1.4x extender.
I would make a few improvements, to make it perfect. 1) If I had a collapsible wide angle zoom with IS, it would rarely come off the camera. 2) I would like to be able to have some more settings for minimum shutter speeds, ISO, etc. 3) Of course, faster AF, dual pixel, etc. would be huge. If you could add a nice EVF similar to Sony's, that would be great, but probably not at the expense of size.
My 2 cents.