I own both an EOS-M and S100. I've had the S100 for a year and a half and just got the M and 22/2 prime lens 6 weeks ago. I haven't used my S100 once since I received the M. As other here have said, the M's image quality is much, much better.
Regarding pocketability, the M is really too big it fit comfortably in pants pockets even with the 22/2 lens mounted. A few times I've taken it along in cargo pants side pockets, but thats not a good solution if you are going to do much walking. I normally carry my S100 in a padded belt pack. I have a LowePro Dashport 20 belt pack for the M+22/2. It's roughly twice the size of the S100 belt pack, but not a huge difference as far as being comfortable to carry with you.
My only issue with using the M in place of the S100 is lack of zoom reach since the only EF-M lens I own is the 22/2. I have the Canon EF adapter and can and do attach my EF lens - but then its a very large kit to carry around. So, if I need reach, I take my M (or 6D) and 70-200 along. At some point I'll probably purchase additional EF-M lenses to give me additional focal length options and still have a portable kit.
I've found the M can replace the S100 for me, so decided to sell S100, its listed for sale on eBay right now.