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.