Distortion: Distortion on crop and full-frame cameras is very negligible.
Sharpness: On a crop camera, center and edge sharpness is very good wide open at F/1.8. By F/2.8, the image is very sharp edge-to-edge. At F/4-F/8, this lens is tack sharp.
On a full-frame camera, the entire image is quite soft at F/1.8. At F/2.8 center sharpness is excellent, but dramatically falls off toward the edges. Like the crop cameras, it is tack sharp from F/4-f/8.
Chromatic Aberrations: Lateral chromatic aberrations (CA) are well controlled. Longitidunal CA (LoCA) however are slightly apparent in the bokeh with green and purple fringing but disappears at F/4.
Bokeh: The bokeh can be a bit challenging in certain situations (foliage, reflected lights in the background, etc). Personally, I find the pentagonal (5-bladed) aperture to be more pleasing than the hexagonal aperture found in the Canon EF 50mm F/1.4 USM lens.
Vignetting: On crop cameras, the vignetting is slight at F/1.8 (~2/3 a stop) and on full-frame cameras vignetting is quite significant (~3 stops at F/1.8 and 1 stop at F/2.8). Both are virtually non-existent beyond F/2.8.