I have been hit in the face by a rock (pebble really) kicked up by my bike. But I always make sure i wear glasses when I ride. Its too easy for something small to get in your eye and cause even momentary blindness. This is definately bad when moving quickly.

I am not horribly worried about too much getting kicked up by a car. Most likely it will end up as a flesh wound, and those build character

(Of course i am in for it now that I have said that )