I have it and I think its a great backup camera to my 5d and have even shot a couple of commercial jobs with it.
For the price and size it makes a perfect backup and takes up little space in a bag.
Aps-c complements full frame nicely as it gives one a different focal length option..ie my 24 ts becomes a 38mm TS on the Eos M
Lack of viewfinder is bad but in 'camera on tripod' situations the live view works well, so good for architecture, interiors studio etc.
Add an optical viewfinder ( I use the Leica one) with the 22mm and it becomes as good as anything out there except the slow af under certain conditions.
Hopefully the firmware update promised at the end of the month will improve matters.
I also have a Leica X1 and the Eos M is competitive on all fronts at 1/3rd of the price.
Really very small and build well, I have already dropped mine and works perfectly still
Is a great walkabout camera and in 'auto everything' mode works superbly well. In fact I think the best way to use this camera is in these auto modes as I have a sneaky suspicion that that's were the camera functions optimally. Go manual and it becomes more difficult. Having said that I use it with a stop down Mf mamiya lens via the adapter and it s pretty easy and convenient. As is using it with the 24ts from Canon.
I think its just going to get better and better and the only serious drawback is the AF and 'AF hunting', but I wouldn't call it horrible....perhaps the firmware update will sort out these problems.
As a backup or supplement it makes a lot of sense and 'normal' EF lenses work well.
As an only or first camera, well there it becomes more difficult and there are some compelling alternatives out there...but the Canon lenses seem to be better than the competition, certainly my 22f2 is a gem and apart from the CA, easily corrected, it is superb...
I have written a couple of reviews on this camera that can be found here at...http://thelazytravelphotographer.blogspot.com/2013/05/eos-m-reviewhigh-iso-noise-tests.html
I image shown here and the crop was made at a jazz concert with the 70-300L at 5000ISO....