the canon 28 is also good. it's less sharp and a couple of stops slower. quick and snappy autofocus though much like with most USMs. the sigma autofocus is actually great too, but it just works differently from canon's so it feels different. it's quick, but it's not the same snap. i dunno hard to explain lol.
sigma has a 62mm filter diameter which is less common while the 28mm has a 58mm which is more common in case you have other lenses with the same diameter.
the sigma has fantastic colours, great contrast and the bokeh is fuggin awesome.
bonus sigma has a long ass warranty included.