I have owned this lens for many many years, initially for use on my Fuji X-Pro1 with fantastic results. I probably paid a measly $25-30 for it way back then. I've compared it in the past to the well regarded Minolta MC Rokkor PF 55mm F/1.7 on the X-Pro1 and found it to be a better performer. In my opinion, it is sharper wide-open, and the bokeh is much nicer to my eyes. Fast-forward to now on my Lumix S1 full-frame, and the results are even better.
Starting with the build quality of this lens, it is superb, made entirely of metal with a solid, compact feel. The MC Rokkor PF 55mm F/1.9 sports a high-quality rubberized focus ring, unlike the Rokkor PF 55mm F/1.7 with a knurled metal focus ring. When mounted via an adapter to the Lumix S1, this combo feels really good in the hand without adding too much weight to the already weighty Lumix S1 body. I will be reviewing the Rokkor PF 55mm F/1.7 on the Lumix S1 shortly, so look out for it.
Image quality on the Lumix S1 is just splendid, wide-open at F/1.9 it is sharp, beautifully sharp with an organic film-like feel. Stop down to F/2.8-5.6, things get better slightly in terms of sharpness while still retaining that film-like organic look, which I love so much with my Minolta and Konica vintage lenses. If you're into bokeh, the Minolta MC Rokkor PF 55mm F/1.9 renders gorgeous, smooth, buttery, pleasing bokeh.
With the results I am getting using the Rokkor PF 55mm F/1.9 on the Lumix S1, this combo has quickly become my favorite 50ish and will be used more often going forward.
All photos were shot in JPG/Raw F/1.9-F/4. LR with minor tweaks to my taste.