Charles Kozierok has posted an interesting thread at FredMiranda comparing the Canon EF 50mm F/1.8 II with Canon EF 50mm F/1.4 USM and Canon EF 50mm F/1.0 L
Crop comparison, near the center-bottom
“#1. The 50/1.8 II has much more DOF than the other two lenses at the same aperture. This probably helps with its reputation as being very sharp for its size and cost. Overall it held up VERY well compared to its more expensive brethren.
#2. Because of the vaguaries of AF at low apertures I have to call the crop comparison close to a draw. What is most noticeable to me is the much greater contrast on the f/1.0L. Also notice how much better it handled the hot area near the letters “TE” in “INTERCOM”
#3. The bokeh comparison surprised me, as the cheapest lens had the roundest highlights.
#4. The L lens seems to be longer than the other two.
Source: Test: 50/1.0L v 50/1.4 v 50/1.8II discussion