I love the perspective of the 50mm lens on a 1.6x crop camera (20D).
AF: Low light focusing is OK. I didn't experience any excessive focus hunting in low light condition. Focusing speed although not as fast as the L or ring type USM lens, it's however, fast enough for my purpose ie. Candid shots.
Sharpness: Soft at f1.4 and f2. CA also present at these 2 apertures. But lens get significantly better from f2.8 onwards.
Built: No complaint here. Well build for a plastic lens.