The IQ is good, for a compact camera, but not even close to my 5D3.
The IQ difference is much more noticeable to "downgrade." Upgrading from Canon aps-c 18 mp sensor to 5D III gave me the sense of "pretty good" IQ upgrade. But after getting used to 5D III RAW files for a long time, the same 18 mp sensor RAW files from EOS-M feels like a much bigger downgrade than initially thought, especially if one is shooting in low-light man-to-man ISO-wise.
With EOS-M, one can still get great results by shooting in good light, or arrange to have more speedlites, strobes, reflectors, etc in low light IME, with a good noise reduction software like Noiseware becoming helpful to narrow the gap.
I am a fan of the 'M', but in no way could I see it replacing my 5DIII. I see the M as a camera on my belt when I might not otherwise have a camera. I am rarely happy with a phone camera shot, even for personal use, so the M is a big step up there.
I see a huge difference in IQ between the EOS M and the 5DIII. Mirrorless will probably replace DSLRs eventually, but I don't see that day on the horizon just yet. I think APS-C is best for mirrorless until all the tech is fully developed.
Once the AF, viewfinder, and ergonomics are mature, then a Full-frame mirrorless with mirrorless-optimized lenses will be the way to go. There's no point of a FF mirrorless on the end of today's 70-200 2.8, or larger lens. Or even the 16-35 2.8. All of that is still over the horizon.