The R adapter is quite interesting. I guess I wonder why now and not before ...
It's just an R to M mount adapter. Leica didn't offer it before (although others do) because there was no sensible way to properly focus an SLR lens on an M rangefinder camera. Now that they have an imaging system that provides the TTL focusing/framing tools, they offer the mount adapter.
... Anyway the other issue is some of them are old designs and how they will hold up, great on a 10 mpx fat pixel DMR but times have changed to a more demanding smaller micron sensor. My bet they will be fine but something to think about and now need to go find them in the deep corners of shops. LOL ...
I'm sure, just as with all adaptations of film era lenses to digital capture bodies, that some will work beautifully and some will be less desirable. People will pick and choose the ones which work the way they want ... nothing new in that.
SLR lenses in general perform better on digital sensors than RF lenses, due to the basic design constraints of the SLR mirror and deeper lens mount registration, so I expect most will do reasonably well. I'm eager to see how well my favorite Nikkors do once I have the new M; they work very well on Olympus FourThirds, Panasonic Micro-FourThirds, and Ricoh GXR-M APS-C bodies so far. Pixel density of the 24x36 @ 24Mpixel format is a little bit lower than 16x24 @ 12Mpixel (36 mm^2 @ Mpixel vs 32 mm^2 @ Mpixel) and both the GXR-M and new M are without AA filter, so the geometry factors of the larger format are the most important influence.