This is going to be a little bit irrational. The MM has been my constant companion for the last month. 24/7. I've got a daily photo blog to feed, for better or for worse.
As an object the MM is my favorite camera ever. And I've owned a lot of cameras (a number of them Leicas but also goodies like a Rollei 2.8F, a Hasselblad SWC, a Contax G2, and Ebony 4x5 . . . I could go on). For some reason the MM has made it easier to sort through my lenses and understand what works and what their limitations are. I think and shoot more manually with it. (In the viewfinder the red arrow on the left is your friend in high contrast situations, the one on the right is your friend in low contrast situations.) I'm making fewer technical mistakes.
It has provoked my to sell most of my non-Leica gear. I'm now questioning whether I need MF.
Jono's advice on never selling Leica gear is excellent.