Conclusion: Vignettes badly at f2.8 and f4. Corners not sharp at all at f2.8 and f4. Good central sharpness at f4, f5.6 and f8. Corners ok at f5.6, slightly better at f8. Nice modeling effect and bokeh quality at f5.6 and f8. Looks worse at f11 and smaller stops - diffraction? Distortion is much less than any of the zoom alternatives. Color rendition and contrast are good at f5.6 and f8. Not a good reputation, so very inexpensive second-hand.
Summary: Reasonably sharp wide-angle lens. There is slight red-green fringing, similar to 15 mm fisheye. Purple fringing is absent. Strong vignetting at f2.8-f4, but this goes away at f8 and higher. Good centre sharpness, but the corners are a bit soft at f2.8, and this improves by f5.6. Gives a bit of a reddish/magenta color cast, e.g. skies look slightly purple, colors are warm. I shopped around quite a while before I bought this lens.