With a DxOMark score of 26 the Canon 28mm ranks very high. It is the second best of all wide Canon lenses and, if you include independently made lenses, it still ranks 5th out of all canon fit wide lenses. The scores for P-MPix are even better, showing that this is a nice sharp optic, scoring 15P-MPix. If you narrow the field further and look for just 28mm lenses, this one is the top scorer.
The individual test results show a lens that is very consistent, the only place where the scores dip is when used with a small aperture when the resolution does fall off. Chromatic aberration is well controlled, distortion is not excessive transmission is excellent and vignetting, while noticeable when using the lens wide open is exactly what you would expect of a 28mm lens.