I bought an M3 with the electronic viewfinder a month ago. So far, I am pleased with the image quality using the 22mm lens and my EF-S 55-250mm STM with adapter. I find it easy to transfer photos via WiFi to iOS devices. Seems like most people have a hard time with Canon's WiFi and apps.
My biggest complaint about the M3 is its autofocusing in AF Servo and burst mode. It just doesn't continue focusing as the subject moves and you continue shooting frames. I don't expect to use the M3 to shoot a hockey game, but its AF should be able to handle moving cars or animals. Dustin Abbott had a similar issue in
his review - see the section with the dog photos.
The build quality could be improved. It sounds like the original M was quite good, but reviews say the M3 is not as good. I've never used the M, so I cannot speak about the differences. The M3 feels alright to me. Not bad, not great. I am concerned about the blue Review button on my M3. It creaks and I do not feel confident about its durability. I think the positioning and number of buttons and dials is just right. Love the dedicated exposure compensation dial.
I'd like to see a small EF-M macro lens. I can use the EF adapter, but when you consider the bulk of that plus the EF-S 60mm or EF 100mm L Macro, I might as well use a DSLR. While pleased with the performance and handling of the EF-S 55-250 with adapter, I just got the EF-M 55-200mm yesterday because of the smaller size.
I have owned a Sony a6000 for a year. It's a decent camera and it does pretty well focusing on moving objects, something the M3 fails at. Sony's E 55-250mm lens stinks compared to Canon's EF-S 55-250 STM and EF-M 55-200. I was impressed by the combo of the a6000 with the EF-S 55-250mm STM on an adapter, even though I had to manually focus. Beside the E 50mm and E 18-105mm G, I'm not a fan of Sony's glass.
I'm not sure if I prefer the a6000 or M3 more. I feel more comfortable with the M3 and find it easier to get good photos with the M3 and my Canon lenses. For my eyes and glasses, the M3's optional viewfinder is easier to use than the a6000's built-in viewfinder. The a6000 lacks a touchscreen (What was Sony thinking?), and I like the M3's touchscreen. However, the a6000 feels more fully-featured and the AF system and 11 fps burst mode beat the M3 handily. The M3 feels like an interchangeable lens version of my G15.
Bernard mentioned the EF to EF-M adapter being overpriced. Not sure where he is, but B&H sells a white box version for $50. I consider that a fair price for a lens adapter.