I went out today with the EOS M, EF-EOS M mount adapter, and a bag of lenses: EF 400mm f/5.6L, EF 70-200 f/4L IS, EF 17-40mm f/4L, EF 85mm f/1.8. As soon as I picked up my backpack containing the lenses I was reminded what it is I like about the EOS M system. I also took a tripod and ballhead to add to my load.
The image quality was fine, but the EOS M doesn't lend itself well to being used with long EF lenses. Composing and focusing a telephoto lens at arms' length while looking at the LCD is difficult, and even more difficult in bright sunshine. The AF is also problematic with long lenses in certain conditions, and I'm not referring to speed. I tried to focus on a dragonfly with the 70-200mm, but just couldn't lock focus no matter how I tried. It kept focusing on the background, no matter what AF options I chose. This would have been easy with my 40D.
If you are going to carry heavy, bulky lenses around you may as well carry an SLR body. You already have a heavy load, so a bit more isn't going to make much difference and shooting will be less frustrating and easier with an SLR compared to the EOS M.
I wasn't sure whether my EOS M system would be good enough to replace my SLR kit for the type of photos that I (personally) take - we all have different needs. It is good enough for 90% of the photos I take, but I still need an SLR sometimes.
I regard my SLR and EOS M as separate systems. The EOS M as a small, light, everyday camera that is easy to carry around and the SLR for more specialised or difficult shooting conditions. The adapter allows me to combine the two to create one system, but I plan to keep them separate. I will always find a use for the adapter, but as I have now decided to keep the systems separate it's not as important as I first thought.http://phil.uk.net/photography/canon_EF-EOS_M_mount_adapter.html