I was a life-long M user right up until the M9 came out. After putting up with the IR-Cut filter fiasco (with the 1.33 crop factor) I was pretty rashed by the time the M9 came out.
So I dropped Leica in favor of a Canon 5D Mk III.
Years later when the second M240-based Monochrom came out I decided to take advantage of the great deals available on the close-outs of the original M9-based Monochrom.
This restored my love for Leica and completely switched back and ultimately landed an MP-240. I'm now fully embracing the Leica M experience again.
So NOW we have the M10. After reading the initial reviews I felt rather cold on the prospect of an "upgrade" - it's thinner, simpler to use, about an ounce lighter, no video (duh), and has better dynamic range. However the sensor is still 24 megapixels, the battery is redesigned and smaller, and I'd have to change my Luigi case - again.
But the clincher is this - I held an M10 in my hands. Now I get it!
The odds are I'll wait until the "P" version is released and the rush on it settles; then I can see upgrading. But until then, I'm perfectly satisfied with my MP-240. One day I may try the video (but I wouldn't bet on it).