My personal experience. I had a Nikon system, a Leica R system, a Contax G2 system and other small compacts. Both the Leica R and the Nikon systems (save my first camera a Nikkormat FTn + 50mm f2) have been sold as I no longer have much time to shoot.
For ease of use, the Nikon system (or the Canon) are the best. If I were a pro, I would use either one. To my customers, as long as they get the photo they want, it does not matter how it is taken and neither do they want to know. My favourite Nikon lenses are the 20mm f2.8 AIS, 55mm f3.5 micro, and AFS80-200mm f2.8. The most disappointing was the 400mm f2.8 AFI.
For the R Leicas, all the lenses are very good to fabulous. The fabulous ones are te 15mm f3.5, the 19mm f2.8, the 100mm f2.8 APO and the 280mm f4.
The difference can only be seen when you blow them up, the larger the better. And when I scan them in, the photos taken with a Leica look more 'real' especially in the shadow areas where they have more detail.
The Contax is better than the Nikon but not quite there with the Leica.
The best photos I have seen from a photographer was an Australian Chinese pastor at the Botanic Gardens 10 years ago. He had a body with 3 lenses that cost him a total of S$200. The photos all came out with a reddish/purplish cast like my 30 year-old Vivitar 135mm. But the mood and the feel of the photos (4R max) was out of this world.
For a lousy photographer like me, the Leica gives me an advantage, slightly better photos with 'nicer' more 'real' colours. A few favourites, all on Leicas, are on my walls except one shot on the Nikon 55mm f3.5.
I am no longer willing to lug all these equipment along especially when I travel. The FX01 with all its limitations suit me fine, but sometimes I wished I had a DSLR along for enlargements. But much as I like the Leicas, I cannot see myself with another bag of cameras, even an M or for that matter a DSLR.
But to me the Leicas rock (or they used to)!!