Here are my two cents on the EOS M + 22mm f/2 STM kit lens. I'll point out that I'm a semi-pro, and I bought this camera as a more casual/portable alternative to my 5D Mark III and heavier L glass.
The main thing I will say about the EOS M is that it's just plain fun. It's portable, it takes great pictures, and has all the pro features I need to give me the confidence that, in a bind, I could leave my big camera at home and come back with perfectly usable shots. Plus, there's just something liberating about using a more casual camera...I love my 5D III, but I seem to set higher expectations for myself when using that camera and it's nice to be able to just get out and have fun.
Anyway, here are some thoughts on the EOS M:
1. Auto-focus - If any one thing could be blamed for the less-than-stellar reviews of the EOS-M, this is it. When I first heard about how slow the AF was, I immediately decided that this camera wasn't for me. It made me think of my crummy old cellphone camera, which would hunt for a few seconds in low light then finally take a blurry shot. I'll say that while the AF can be slow, when it *does* lock -- unlike the cell camera -- you WILL get a great shot. Furthermore, you can maximize AF performance by using the "FlexiZone - Single" mode, setting the AF Operation to "ONE SHOT AF", and disabling Continuous AF. I'd also suggest leaving the "Touch Shutter" option disabled, but that may just be a personal preference. [Note: Supposedly the MagicLantern folks are working on homebrew firmware that will fix some of the AF issues, but I would not recommend buying any camera based on the vague promise of some future update.]
2. Image Quality - Image quality is great, but it's no 5D Mark III. If you're expecting 5D3 level images from this then you're in the wrong place, but with some care it can produce shots a pro would be proud of. What I noticed most when compared against the higher end cameras would be contrast range and saturation. Some of this could just be the lens, but I haven't yet had a chance to put on something like my 24-70 f/2.8L or 50mm f/1.2L. I plan to do that soon and will update the review with my results. If you shoot in RAW you'll be able to overcome a good portion of the contrast/saturation problems with Photoshop and CameraRaw, but even so I've had to be more careful with blown highlights (particularly specular highlights) on the EOS M.
3. Video Quality - I'm generally impressed with the video performance. That said, expect to use manual focus if you want anything near professional-level video. I haven't tried this with a focus peaking monitor yet, but I'm not even sure the camera would be fully functional after losing the touchscreen.
4. ISO Performance - The ISO performance is pretty good. Again, it's no 5D Mark III but up to at least ISO 1600 it's not something you can't fix in Photoshop. My biggest complaint with ISO is the fact that you can't seem to use anything finer than full stop increments. Am I missing something?
5. Touch screen - I had mixed feelings about losing the majority of my physical buttons and switches in favor of a touch screen, but it is growing on me. The settings I use the most are all easily available via the screen (shooting mode, aperture/shutter speed/ISO, exposure compensation) or through physical buttons (menu, quick menu, exposure lock, drive mode, etc.). One thing that took me an embarrassingly long time to figure out was how to zoom on the image preview screen...using my fingers to pinch and zoom seemed completely foreign coming from the 5D3, but it works well.
6. No view finder - This doesn't bother me as much as I thought it would, but I do embarrass myself by occasionally lifting the M to my eye.
7. Features - I love the fact that, feature wise, this is very similar to Canon's high-end DSLRs. I had this little camera attached to a wireless strobe system in my home studio, and it just worked. The user interface was familiar, the settings and options were similar, and the results were fantastic. It's also got all the shooting modes (M/Av/Tv/P) that a pro would expect, plus the full auto and "scene intelligent auto" settings that allow you to hand the camera off to a random passerby to get a family shot for you while on vacation.
8. Ergonomics - The camera is small and light, and I can stick it in my jacket pocket and "almost" forget that it's there. That said, the small size makes me wonder how easy it will be to hold with heavier glass attached via the EF/EF-S adapter. I'll provide info on that when I get around to it as well.
In short, as long as you don't come into the game expecting pro-level performance and quality, you will not be disappointed. It does exactly what I got it for: it gives me a way to leave my 25 lbs. camera bag at home on occasion, while still delivering great shots that I'm not embarrassed to share online. Plus, for someone who's already invested a great deal of money on Canon glass this makes even more sense. Sure the AF is slow, there aren't many dedicated lenses for the M mount (yet), and it's not a replacement for a high-end camera...but I didn't go in expecting any of that, and so I came out happy.
I've had a chance to use some of my EF lenses on this camera, and here are my results:
1. Canon 16-35mm f/2.8L - Good focal length range for a cropped sensor, AF speed comparable to the 22mm f/2 STM.
2. Canon 50mm f/1.2 - AF speed comparable to the 22mm f/2 STM, aperture opens all the way to f/1.2 if you were wondering.
3. Tamron 18-270 Di II VC PZD - AF is unusably slow, don't bother.
4. Canon 18-135 f/3.5-5.6 IS STM - A great pairing with the EOS M, this is on my camera 90% of the time.
5. Canon 100mm f/2.8L IS Macro - The EOS M is great for macro. The cropped sensor gives you that extra bit of reach, and the small size and light weight make it easier to maneuver into tight spaces. I'm having a bunch of fun walking around outside my house with this setup.