My early Summilux, which I've had since the early '70s and truthfully haven't used all that much, has given me a higher percentage of amazing images than any other lens I've used. Most of those have been shot wide open, in available darkness. I think that handheld camera steadiness is enhanced below about 1/30 sec, when the image remains visible instead of blacking out as with an SLR; most of the Summilux's best shots have been wide open, all handheld, at 1/15 or below.
I used a Canon f/0.95 for several years in SE Asian photojournalism. There were times when a flash could NOT be used---I always hated to use one anyhow---such a photographing Western pedophiles with children in Manila's red light district, when the Canon's low light (low life) ability might have been a literal life saver, as they apparently did not believe I could get pictures of them in the dark. I did (thank you, Diafine!).
The little plastic Nikon 50D f/1.8 currently gets consistent use as a fast, cheap, light, and high-performing short telephoto on my DSLRs. For some reason, it seems to focus better automatically than I can do with the old, beautiful manual focus 50mm f/1.4s. I need to figure out why manual focus lenses back focus on my D300, because they certainly do.