5D3 + EOS M + L Primes = Ultimate Setup in my Book.
I don't know. I think I will hang on to my T2i as the "compact" backup to my 5D III. Once you add the EOS lens adapter and stick an L glass, the EOS M's advantage in portability decreases quite a bit over Rebel bodies, who have the advantage of EVF and built-in flash, even wireless external flash control for later bodies.
Once the next generation of hopefully better APS-C Canon sensors make it into the EOS-M MkII or whatever, with EVF, then I may seriously think about picking one up.