The Defeet Slipstream shoe cover was developed for two purposes: to shield your feet from chilling cold and to create an aerodynamic advantage. Simply wear the Slipstream shoe cover directly over your cycling shoes to feel greater comfort on cool days and to slice through the wind.
However, more importantly for us, wearing a thin overshoe that improves the look of your feet on the bike in cool, but not freezing weather really makes a psychological difference to the ride. Ok, so when it's cold and wet you should wear a really good pair of weather proof overshoes, but for milder conditions, keep those feet to temperature and really look the part with an overshoe (or maybe 'oversock') like the Defeet Slipstream. Like shaving your legs in Spring / Summer, it really makes a big difference to your on the bike attitude.