Nikon and Canon users pay attention: just because these are Leica M shots doesn’t matter so much; both Nikon and Canon (and Zeiss) have great lenses for similar effects.
On my early July photo tour to Yosemite, I loaned one participant my Nikon 35mm f/1.4G, and he was so taken by the newfound capabilities at f/1.4 that he ordered the lens immediately after the photo tour.
This example was shot at f/0.95 at 1/60 second at ISO 80 using 9 stops of neutral density filters at about 10:30 in the morning.
Not every shot has to be wide open. Here the Noctilux was shot at f/5.6. This is Tioga Lake, and as of July 6, 2011, it had still not iced-out, though it was clear two days later.