I've been using a 1950's era 50mm f1.8 Canon Serenar on my Leica since 1999 or so and lately I've been worried that I'm not getting as much out of my camera as I might be and was luckily able to test a mid 1990's pre-asph Leica Summilux side by side with the Serenar.
There's definitely a difference wide open in the bokeh -- the Summilux is a lot smoother. It's also half a stop brighter and the color is a bit different. And the Summilux focuses closer, which is a distinct advantage, though it's also significantly larger and heavier.
In the 100% crop it seems a bit sharper but at this aperture it could also be a focusing variation as well.
My ultimate conclusion is that the Summilux, while nice, isn't $1,200 nicer than the Serenar.