I came from Canon 5Dii and Nikon D700, a typical upgrade for many new Leica users.
Lets first get over the main downsides from a DSLR:
a. Lack of wizzy buttons to play with.
b. Focus dichotomy: Slower to focus short range (very slightly), although faster at long range for me and a lovely spin to infinity for landscapes or sort-of distant objects.
c. No live view: Would be handy for tripod work and waist level shooting/framing.
Ok, so now for the upsides:
1. Never out of focus
2. Choice of exactly where to focus - e.g. on a DSLR, when the focus spot is bigger then the eye, is it focusing on nose, eyebrow, eyeball, etc.
Here you can see the precise focus on the eyeball brings out the reflections on it, and therefore a nice glint.
3. Sharpness combined with lovely definition:
4. Superb colours. The best I've seen yet:
5. Real world lovely dynamic range. I can combined the sky with my buildings at last:
6. Leica lenses really are super-sharp wide open. The ability to use f1.4 on a FF camera lets me ignore the objects I want to, here the glasses in the foreground are blurred out and the subject is still super sharp:
7. No AA filter and superb micro-lens quality prevents bleed from extreme contrast areas of the scene. Here the B&W poster is B&W, the art object is starkly coloured and the viewer is naturally coloured:
8. Rangefinder viewfinder allows you to frame whilst viewing outside the main subject and thus provides more latitude for focus and re-position. Here I caught the girl jumping in the foreground:
9. The lightness of the camera means I take it everywhere and don't miss a weird moment:
10. Much is talked about the sharpness wide open, but tiny apertures are super sharp as well:
11. Oh, and the low light/high ISO performance is excellent:
13. The B&W rendering is, of course, superb:
14. The unobtrusive nature of the camera (even with the red dot!) makes it subtle to use:
15. Well there are many more I could show, but enough for now. I'll just slip in how I love how it makes your loved ones look:
16. The useability of the camera is great. I love to spin the aperture dial and know exactly all the positions for lenses.
The eyesight in my right eye is not as good as my left. However I still use my right to focus. What should be remembered is that focus is a relative, not an absolute thing, it just has to look the sharpest (and give a contrast pop).
Overall a very enjoyable 6 months. There are many great developments in the compact system camera arena, but nothing in FF with the lens quality on the horizon to even challenge the M series.
I would predict never any challenge with a brass top :)