For pilots, non-polarized sunglasses are the rule of the day. We fly in all inds of weather, and as anyone who has ever travelled on an airliner knows, airplanes normally get on top of the clouds when the weather is poor. And this places pilots closer to the sun, exposing their eyes to more intense radiation than you will experience on the surface of the earth.
The reason pilot do not want to use polarized sunglasses is that polarized sunglasses filter the light and block out any reflections off of glass or metal objects. When you are flying an airplane it is important for you to see any aircraft that may be on a conflicting path, and sunlight glinting off of an aluminum airplane can help keep you safe.
In fact, the Federal Aviation Administration has produced a document outlining the reasons pilots need to use non-polaized lenses. This interesting document shows how important sunglasses are for pilots, and details why non-polarized lens sunglasses are the only kind pilots should wear.
On the other hand, polarized sunglasses are important to fishermen and some participants in sports like skiing because they filter out the reflections which can otherwise impede your vision. For fishermen polarized sunglasses allow them to see into the water so they can see the fish, and for skiiers it allows them to ignore glaring sunlight off of the snow.
For drivers, an argument could be made either way. While sunlight glinting off of other cars or the glass on buildings might alert the driver to some danger, non-polarized lenses would provide a much clearer view of the road ahead for the driver.
In each case, however, it is important to get sunglasses with scratch and impact resistant lenses, frames that are well constructed and which fit comfortably on your head.
As a pilot I recommend Randolph Engineering aviator sunglasses to other pilots.