I've had my M8 for 12 years,
With Leica- they do not understand digital.
My primary interest when it comes to new camera models is ISO performance. It's the dominant (by far) concern I have with my personal work and the occasional paid gigs I used to do. I'm confident in producing decent images with all of the previous models as long as (lack of) ISO performance doesn't get in the way.
I've found that the M10's ISO has been performing quite well though. With the M8.2, M9-P, and M240, I certanily felt that the ISO was just not performinig well enough. The M10 has been fine so I haven't felt the need to jump on the next model.
The only other gripe I have with the M10 is its dynamic range, it could be better. Weirdly enough the M240 seems to have better dynamic range than the M10. But this is just a minor gripe.
With those earlier models, I just won't go above base iso, sometimes even at night.
My CCD M-E 220 .... When it does eventually brake, then it'll probably be an M10 for me.