I was recently biking down Wilson blvd in Arlington, and a car turned directly into the bike lane in front of my group. A group member used their loud horn (http://loudbicycle.com/), which caused the car to immediately stop their right turn, and swerve out of the bike lane, letting the group pass safely in the bike lane. Honestly, without that horn there is a good chance at least one of us would have made impact with that car.
I don’t have the handlebar space for one, but I after seeing that, and hearing how much it sounds like a car horn, I fully support their use in urban settings.