after seeing the great pictures from Dustin and others with the Helios 44-2, I started looking for one on ebay and ended up with a SMC Pentax 50mm 1.4 K-mount instead. I must say: I am impressed. It produces great bokeh and as far as I can tell is pretty sharp wide open and only costed 85 Euro.
It is fully manual and it is a challenge to nail focus. Luckily, I was able to change my focusing screen to a EG-S, which really helps and the view finder in the 6D big and bright.
The build quality is really impressive and its a joy to use.
Of course, if you are reading this you'll want to see some pictures.
The following pictures are all on a 6D and all @1.4. They were shot in RAW and run through Lightroom with the default settings.
The first two are at 200 ISO and 1/200.
The third one is at 250 ISO and 1/125.