I've got my M on order (backorder) from B&H. I am actually amost more interested in the M42 adapter and seeing how my legacy glass performs on it. For example, I love my Super Takumar 35mm f/3.5, but it is the one lens in my vintage kit that just doesn't hardly work on FF. I loved it when I had a 60D and even a 500D, so I'm hoping to get some new life on it with the M.
There is one area I wish that Canon had matched some of the mirrorless competitors in - in body stablization. I like the system the way it is on DLSRs, as the best in lens stabilizers seem much more effective than in body stabilizers, but on something like the M (and with my vintage lenses), some stabilization (even if just a couple of stops) would be huge.
But other than the shorty forty, I think I would find that the additional bulk of the adapter + a big lens would kind of defeat the purpose of the M. My 6D's work just fine, thank you very much.