I have not spent a lot of time with the M3, but these are my initial thoughts after setting it up and trying a few lenses.
First IQ is fine as expected. It is a 24 mp sensor from Canon, it should be.
When ever I get a new camera I spend a couple hours going through every part of the menu to set things exactly to my liking. The lack of custom settings was the biggest surprise for me. Coming from a 70D and 80D the options on the M3 are very limited. I guess Canon feels that this camera is for people who don't care about this things, and would be easily confused by things like "auto iso" settings.
The ergonomics are fine, for being such a small camera it is pretty comfortable to hold.
The buffer is SLOW. I would barely call it continuous shooting. RAW+jpeg fills the buffer with 4 shots. Not that the servo af would keep up anyway.
Now the other surprise. Auto focus with the EF-M mount and EF lenses looks to work just as well as the native M mount lenses I used. Which means pretty slow. The af with the 100-400mm IS II adapted was MUCH better than 22mm f2. Although af was fast enough with the 100-400mm, it didn't seem very accurate when reviewing photos on my computer, although I would need more time to really test this and confirm.
Price was good. I got it new with the 18-55mm, evf, and EF-M adapter for $600.
Pros- image quality, price, size, works well with EF lenses
Cons- weak menu settings, Slow buffer, sluggish af
Conclusion:
It is mostly what expected. I wanted something pocketable that could also be a back up in emergency type situations. I sold an Olympus e-m10 ii to get the M3, and in all honesty everything about the Olympus is far superior except IQ (on par), size because of evf, and use with EF lenses. So I am still on the fence. I might be better off keeping my 80D around for back up and just getting a small point and shoot.