Like all other companies Canon will place the M cameras in their lineup in such a way that they won't cannibalize anything else. This means that - even if they could make it so - the M series will never be as good as a DSLR except possibly at the low end DSLR, high end M. Canon probably decided to do the line because of competitor pressure, and it provides a stepping off point into DSLR's.
So for an existing DSLR owner why would we get a M, other than for the cool factor? As a smaller body complement to our existing system. We can use our existing lenses, but if we do so then it kind of defeats the purpose of having a smaller complement to the system. Anything above the 40mm pancake will be a monster on that body, and you have to use an adaptor.
So ... using it for it's best purpose, as a small side camera for those times you can't bring a DSLR, look at this
Here we have a DSLR with a pancake next to the M. Sure the M is smaller and lighter, but is it that
smaller? I see maybe an extra inch vertically and horizontally at maximum. Indeed, when I put the shorty-40 on my 5DmkII I have a really tiny, lightweight camera. I barely notice it, and I have the full power of the existing DSLR (speed, focus, IQ, features, etc). I don't have a hugely quiet
body, but if I had a 5DMKIII then I could use quiet shutter mode.
So ... EOS-M isn't for me. The small reduction in size isn't large enough to warrant the price tag and extra equipment. For the cost of an EOS-M you could get a duo of pancakes, the 40mm and Voightlander 20mm.
I got the Fuji x100 as a side camera for those times when a DSLR wouldn't work. There are a few occasions where it's great, so small and unobtrusive (and quiet) that the DSLR would have been too much. Most of the time though it sits unused.