I recently got buzzed by a van who overtook at high speed on a blind bend and swerved across me to avoid a head on collision with an oncoming car, nearly wiping me out in the process. I yelled as he did it and he stopped to confront ME because I yelled (he then tried shouting at me) - I was in no mood for his BS and unloaded in no uncertain terms telling him about what I thought and that his actions were on video. He went from aggressive yelling to nearly in tears in a split second and tried to blame the oncoming driver for driving too fast - I made sure he knew what I thought of that excuse. He didn't quite know what to do and when he eventually pulled away he drove the next mile or so at about 20mph! Hopefully he will think twice before doing something that reckless to a cyclist again.
I don't go looking for trouble, but if someone does something genuinely dangerous I have no problem in telling them.