I had the XR Rikenon 50mm f/2 for a while and ended up selling it on, not a keeper for me. Image quality is really very good for the price you pay, but the build quality is definitely lacking.
I ended up picking up the XR Rikenon 50mm f/1.7 very cheaply and that was my fast 50 for about a year, until I moved to the Pentax FA50mm f/1.4
I found the XR Rikenon 50mm f/1.7 superior to the f/2 in every way. Definitely sharper, better colours and better build quality. Both the f/2 and f/1.7 were in equivalent optical condition (clear glass, snappy aperture, etc.)
Here's two samples from the 50mm f/1.7: