BMbikerider said it all.

I use a F/2 50mm Sekor with my M42 Praktica camera and I have no complaint. It is not better thean the average 50mm but not worse neither. I did not checked its used market value recently but I bought mine $17 several years ago which was a bargain compared to other M42 standard lenses (Zeiss Jena, Meyer, Takumar, etc...).