I am not sure how the 40mm pancake will work on the EOS M. The focal length would probably be too long for my tastes on a crop body, as I never liked a 50mm prime on a crop and instead used a 28mm f/1.8 as my "walk around" prime. It's a very usable length for me on the full frame 5D3, though. Since you do a lot of portraits at your conventions, though, it may work for you, Patrick.
The image quality is quite good for such a small, inexpensive lens. It's pretty sharp wide open. The lens does vignette a bit when shooting wide open with a full frame camera, but that is easily fixed in Lightroom. I am not sure about vignetting on a crop body, as I have not tried the lens on my 7D.
Trying to compose using only the back LCD of the EOS M would drive me crazy. When we were in Glacier last week, I took only my 7D + 100-400L on one of the trails (we were hiking this particular trail primarily looking for wildlife), and discovered that I wanted to take some landscape shots. So, I resorted to using my wife's Powershot S100, which she had in her pocket. Trying to compose using that screen in the sunlight was not fun.
Edited by Scott Merryfield, July 24 2013 - 04:17 AM.