Tested on a Sony a6000. Sharp in the center wide open, very soft in the corners at 2.8, and does not really improve as expected when stopping down, at F8 the lens reaches its peak performance. Colors and contrast are great! Aperture and focus rings are smooth and precise, the overall build quality is fantastic. This is a great lens when taking in consideration it’s price, it’s better than the Pentax M 28mm 2.8, is sharper wide open, better colors, better contrast and less distortion, but the Pentax tends to be cheaper as well. I paid 60€ for this lens, and a good deal is to pay no more than 75€.
I believe the Rolleinar MC 28mm 2.8 shares the same optical formula and thus it should yield very similar results.
There are not many alternatives for this price-performance ratio, but if you want a better 28mm performer you should check the Zeiss 28mm 2.8 for Contax, that is a much better lens with a higher price tag (300€ for a lens in a great shape).