I'll admit to using an AirZound horn on my commuting bike. For a long time I had nothing, but I got fed up with motorists pulling out on me, and often driving off without even being aware of what they'd done.
Most notable occasions I used it was:
1. Learner pulled out in front of me on a roundabout, while being supervised by a professional instructor.
2. Bin lorry pulling out on me. The roads met at an angle, so the driver couldn't see, and was being guided by a crew mate in the passenger seat. Fortunately the driver heard my horn and stopped. When I glanced over my shoulder after wizzing past, the driver had his hand over his heart, in shock that he'd almost squished a cyclist.