i actually prefer using the M over my 5D with legacy L lenses for a number of reasons, and to be quite honest, am quite surprised how much that little camera has grown on me.
1) I use MF almost
exclusively (for both still and moving subjects), and very much prefer the way it works on the M along with the touch screen
2) I shoot with a tripod, so the space and weight savings is profound when I have to carry things for any distance.
3) I shoot low ISO, usually no higher than 400, so crop-sensor-noise doesn't bother me.
i dunno, it just works for me! my 5D is set aside as my backup
(but i imagine i'll bust it out occasionally when i need razor thin DOF...)