It seems a lot of people are missing the key innovation here. It's not just a loud horn, it's a two-note horn, designed to sound just like a car horn. It's about kind of sound, not just the volume.
Drivers react instinctively to two-note car horns. They don't generally have the same instincts developed for bells or air horns.
As a New Yorker that relies on my bicycle for daily transportation, I can't wait to have one.