Same set up, 5DIII is my primary camera and I bought the EOS-M a couple years ago.
I use it when I want a small and light camera, when I want a "second body" (say, when shooting birds, have it for the occasional wide-angle shot), or when I do not want to stand out as "a photographer." ...
I've been impressed by the IQ. It could be better and I honestly hope that Canon releases a very good M3.
But, I do use my 5DIII >95% of the time.
+1 Overall, I'm happy with mine. Like most of you I use it as a 2nd camera to my DSLR to use when a DSLR rig either isn't allowed or would be too large/conspicuous to take with me. I used to own a P&S S100 for these purposes and the EOS-M is a much better camera.
We took the kids to a theme park last fall and I took my 6D + 70-200 2.8 II on a black rapid strap as my primary camera and decided on the M with the 22/2 lens mounted for posed wider angle shots and for video. Worked great! I didn't have to mess with changing lenses and the M is smaller and lighter than my 24-70 2.8 lens (I didn't own the 35/2 IS lens at that time).
3. Always a camera with me- It is compact and inexpensive enough for me to always keep it in my car- this allows me to have a camera around if some unexpected photo opportunity comes up; for example, I ran into a dragon boat race in a place I had gone to attend a management course.
The M also serves the "always with me role" for me. I keep it in the backpack I take to/from work every day, so its there if I see an interesting situation during my commute or for a few quick pictures at work. I also take it with me on business trips in case I run into something interesting. Most of my business trips don't include much if any free time, so its typically less than 10 shots per trip, including some out airplane windows.
I also use it as my backup camera with the M adapter. Thus far, I've never had to use it in this role since my 6D has never failed, but its nice to know I have another camera option on vacations or outings just in case.
Overall, I'm pleased with it. I would not be happy with it as a primary camera. We have a toddler and there is no way the focus can keep up with him unless he is asleep. My entire investment in the M system (body, 22/2 lens, 18-55 lens, M adapter) is under $420, so I feel its a bargain for what I use it for.