I don't care about other people's cars, I don't like any kind of decal on my cars. I like a sleek, clean look and think that decals take so much away from a car. But that's just me speaking from a car modding perspective.
I only have two stickers on my car. They are window stickers. I have white Air Force wings on my back window like this link but mine are white. They are for my fiance who is in the Air Force. Then I have a pink browning deer on my back window on the driver's side. link
I think window stickers/bumper stickers are cool. I'm not a fan of the all-over look, but a few to express who you are are cool.