The EF 20/2.8 is certainly not an exceptional lens. It performs relatively poor if used wide open, resulting in soft and darker corners (Canon 5D full frame sensor). To obtain very good results, one has to stop it down to f/8 or f/11. Thanks to the high-ISO capabilities of the 5D, and to the reduced impact of camera shake at slower exposure times this is not so much a problem. Therefore, I still use this lens, accepting that it performs best only at the smaller apertures.
Flare resistance is OK, distorsion is also not so bag (especially when compared to the 24-105 at its wide end). The shade is very wide of course and, thus, has only a limited effect on both flare suppression and protection of the front element.