As you have discovered, the 100mm APO Macro is one of the greatest lenses I have ever used. It excels in nearly every area -- optics, handling, bokeh, build quality, versatility. The 120mm APO Macro for the S is even better, but very similar in spirit. It is also a lot bigger and heavier.
Glenerrolrd has some very good information, and I can back up what he says. His choice of lenses for the R system is spot on, though not all will be like the 100mm in performance...perhaps only the 180mm 2.8 Elmarit, which is another stunning lens. The focus is strangely smooth...in fact, perhaps even a little too light for my own taste, but it is very quick to use for a lens of its focal length. Again, optically, it has few equals. I also very much like the 28-90 zoom, which I actually bought from Guy Mancuso. I mostly used it on the DMR though, so I am not sure how it stands up to modern bodies.
Another thing to consider is that the R lenses, as SLR lenses, will inherently work better on mirrorless cameras than the M lenses. So while they are in a way stuck in time, they have the advantage of being more "native". The M system has its share of superb lenses, but they really seem to perform at their very best on M bodies. If you really want the best experience of Leica lenses on mirrorless cameras, it seems like the L mount lenses on the SL, S1 or S1R are where it's at. I just purchased an S1, and while I do not have any SL lenses yet, I tried the 75mm APO Summicron in the store and it felt and looked amazing. The focus was very quick, bokeh exceptional, and sharpness seemed outstanding as well. Of course, it is not going to work on your Nikon...
So far I have been using M lenses, and while I have not had a chance to make a setup to compare to the M10, so far they seemed to be performing quite well...far better than on the Sony, where I found them unusable for most of my landscape work, apart from the longest M lenses (75mm and up).