I have had the M246 now for 4 months. I had the MM for 3 years.
I am overall pleased with the choice to change from the MM to the M246, but with a few caveats. While I like the new shutter and build of the 246, and have adjusted to the slower start up speed, I liked the feel of the MM just fine. Files process quite differently, in my opinion. The M246 renders a "creamier" look (almost like it has more IR sensitivity, as skin tones glow a bit more), that does well with modern glass, while the MM seems to render a harder look that works very well with older glass.
The M246 seems to have a cleaner file for a given ISO, and it takes a bit more sharpening to have the M246 files pop. I got very used to the MM and put many many images on it, so I am biased by that experience, but I do feel that the MM files at first render with a bit more pop than the M246 (similar-ish to the M240/M9 differene), but the M246 files end up having a bit more richness as well, and ISO's can be pushed comfortably to ISO 6400 (compared to ISO 3200-5000 on the MM).
What's also interesting to me as the M246 files seem to be less responsive to the B/W Light/Dark sliders in LR, so it takes a different effort to make things work well. But I am still learning much about the 246, enough that I have not posted many shots with it so far...though I have taken my share.
I have also noted that it's a bit more challenging to easily hand hold to 246 compared to the MM and get a pristine/sharp image...maybe a weight thing or an 18 vs 24 mp factor....
Ultimately, I have found that the MM would be my choice if I were an old glass guy (Leitz glass), and the M246 seems more tuned to performing well with newer glass.
I feel that I was able to coax a bit more out of the MM than the M246, but that being said, I have a long way to go with the 246, and the journey is only beginning for me, in many ways....