I have a Hornit, it's so loud and I've used it with great success many times. It really helps me when drivers are turning into my lane and can't necessarily see me due to obstructions (like parked cars, trees, etc.). I really like that I don't have to worry about pumping anything up (runs on batteries) and it has a trigger that can be placed wherever is most convenient for your finger to push it.

Downside: the louder of the two sounds that it produces is unrecognizable to most people, unlike the airzound which clearly sounds like a horn. The Hornit just sounds like a crazy loud and fast siren, and not everyone will react to it in a way that you want or react to it at all. However, I've had way more successes than failures with it so far.