I have several 50mm Pentax lenses, M 50's and a A series f1.4 that is my favorite. All of them do well, the M 50 f1.7 odes very well, and a couple of f1.4 and f2 lenses also get good results. The A 50mm f1.4 is the one I almost always grab though. The only Sears lens I Have is a 80-200 model 202 that fell apart one day, literally, and i oculd never get it put back together. After seeing what it could do, I wouldn't hesitate to try out a 50mm Sears lens.
Here's why I like the Pentax A 50 f1.4...
with extension tube (Rokinon teleconverter with glass removed)
Just the lens by itself
Both taken with the SMC Pentax A 50mm f1.4, hard to beat results like that.
Very sharp, fast enough for low light, although I rarely have to go wider than f8 except for sunsets, (and I shoot almost all ISO 200) and pretty nice bokeh too. To be fair, my other f1.4 and f1.7 M 50's do a great job too, I pull them out now and then for a few shots...