I've been using one of these for several years. It is a *very* effective tool for getting cars to notice a bike; also the occasional pedestrian or even another cyclist. It does not include a force-field, so the horn-equipped rider still has to leave an "out". But where there is a little wiggle room, it really gets attention. The most spectacular reactions have been from cars jumping their turn at 4-way stops, cars not stopping for a right turn (coming from my right) when I have the right-of-way, and cars merging into my lane. They really don't expect a noise that loud from a bicycle, and I think they may suffer a momentary doubt about whether there is another car they haven't accounted for. The reaction is usually a hard application of the brakes.

I have mine on the handlebar of my commuter bike, where it's easily reached with my left thumb. I can brake and use the horn simultaneously. I keep it pumped up to 80 PSI using a floor pump I keep at home. Sometimes I go for 2 weeks or more between topping-offs; sometimes I have to pump it up every couple days.

My only complaint about this device is that it doesn't work well in the cold, even when it's fully charged. So I don't put it on my "winter" bike.

It's a very useful safety device.
Steve