This is probably restating the obvious, but w/ my photos there is a very noticeable difference between Leica R glass and Nikon, and I've used some expensive Nikon lenses. People are driving up the price on the R lenses so they can shoot it on their DSLR's w/ an adapter. No one is buying adapters to use Nikon lenses on their DSLR's. This would seem clear as glass to me.
Leica has a monopoly on elitism, not quality. People are adapting Leica-R, in a lot of instances, because of imagined, or subjective 'advantages.' Sometimes, just to be different. Sometimes, just to muck around with another gadget. No one's adapting Nikon lenses to Leica bodies because there are no Leica digitals (current cameras), and no one buys an R-body for any reason other than to use a Leica lens. The bodies simply have no real advantages.
There's a lot of BS and voodoo associated with Leica. People will crow about the quantitative performance until it's pointed out that the MTFs don't correspond to the raves. The Leicaphile will then claim the Leica advantages, conveniently, can't be seen in charts.... Or, when a Nikon/Canon lens is soft(-er) at a certain aperture, it's inferior. When it's the Leica that's soft, it's non-clinical. Atmospheric. Or, it 'draws' in a more pleasing manner.....
I've owned Leica R7 and R8. I've had the 28 Elmarit (latest), 35 and 50mm summicrons and 80 Summilux. Among those, only the 28mm was superior to other lenses i compared, and even the 28 was only equal to the Contax 28. The 35 Summicron was no better than a Canon 35/2, and both were easily beaten by the 35L. The 50 Summicron was not better than the Canon 50/1.4, unless you want to compare apples to oranges and say that wide open it was better than the Canon wide open. But, when both were at F2, i preferred the Canon, actually. The 80 Summilux was trounced by the 85L.
I believe there are imaging qualities that people may LIKE from the R lenses, but it doesn't mean they're superior. I like a particular shot i have from the 35 Summicron. Nice bokeh and such. But, that's not brand-exclusive. When people get a nice result from a Leica lens, they tend to apply it to the marque. I've used Canon, Nikon, Contax, Leica-M, Leica-R, etc. in 35mm and have pictures i like from each. In no way can i say one brand is more responsible for better images than the others. They're just different, and each lens is different. If there were an empirically, universally superior platform, we'd all be idiots for using something else.
In general, i don't like Nikon's current primes. So, in that respect, i'd rather use Leica R. But, that's just re: AF lenses. After spending thousands on such my current favorite lens is a Nikon 50mm 1.8 Series-E for which i paid about $35. It all depends on what you want to shoot and the imaging qualities you most value.
As often as i've read some comment about a Leica lens having richer, more saturated color, i'll look back at the Leica shots i like most and they'll be subtle, with lower contrast and saturation. It's kind of ridiculous to make these broad claims and assumptions. If you only want to shoot one kind of subject, one way, with one film - sure, try to match the lens to the purpose. Try. But, that assumes you're so single-minded that you can/want to ignore all other options, variations, opportunities.