I guess because my bike is considered a tourer, Kawasaki installed a fairly decent horn that sounds more like a car horn (import, not domestic). But I have rarely ever used it.
I especially haven’t used it in crossing situations, as my hands are usually busy getting ready for avoidance. Plus, I’m skeptical about whether the car driver is going to be able to quickly located the source of the horn. If he doesn’t already see me, he may just assume the horn is someone behind him.
One place I have used the horn is when I’m along side a car that is starting to creap into my lane or puts on his blinkers for my lane in cases where I can’t immediately accelerate out of the way.
I also never use it as follow up to someone doing something stupid. The way I look at it, if someone made an honest mistake — they usually get it once they see me on their bumper or sliding past at the next light. If they are idiots and truly oblivious — me and my horn can’t help them.