Canon M Mount Adapter lets you use Canon EF and EF-S lens on Canon's mirrorless M series cameras, it's Canon's way of expending the choices of lens for their relatively new mirrorless market, which is good in some ways.
Below are some points that I think is going to help you decide if you need this adapter for your Canon EOS M camera:
Solid metal construction that you know going to last. I think that's one of the reason for it's high price, SDG $150 at the time I got this. Although personally I felt it's relatively expensive for something like an adapter, but I suppos it's partly due to the market for Canon mirrorless camera are still relatively small, so small production equal to higher cost.
All data comnication from the mounted lens and camera will perform as usual, aperture, focal length etc will be recorded by your EOS M camera. Auto focus and Image Stabilizer will work if your lens provides these functions.
Through my own personal test I find that the auto focus is much slower on my M3 as I try to illustrate in a video below:
I'm not too sure it's because of the limatation of my M3 focusing or the long focal length play a part, but clearly the focusing speed with a native M lens focus much faster in the same environment compare to a lens mounted with the adapter.
Mounting an EF or an EF-S lens isn't very pracital as the whole camera will feels unstable in weight distribution and not easy to use in some ways.
At this point, I would rather use my DSLR if I am using it for taking photos. But if you are capturing video, the built-in touch focus and continuous auto focus tracking found in EOS M3 is lacking in most of Canon's DSLR, that's where the adapter become relevant. Mount on a tripod or monopod, you greatly expands the perspective and creativity of the video capability EOS M3 could offer.
If you use your EOS M camera mainly for capturing videos and is looking to expand your perspective and creativity with your EF or EF-S lens, this adapter will definately serve that purpose. But if you use your EOS M camera primarily for capturing photos, you will more likely stick with your native M lens as mounting a big lens on a small camera body just isn't very pratical and the slow focusing doesn't really helps either.