AirZounds horn is great
I highly recommend the AirZounds. I'm on my third. The first lasted about 6 years; the second, with an aluminum canister that could be pumped to 100 lbs, instead of 80, died in a garage fire; and now a third. The Schrader valve on the first two was a bit too short. My floor pump wouldn't work, so I had to use my Topeak Road Morph to fill it. The horn simply does not leak; it will stay at full pressure for months if you do not use it. My guess is that one fill is good for 20-30 2 second blasts.
The horn mounting leaves a bit to be desired. It is not possible to use both brakes while pressing the horn button, at least with my set-up. The canister need not take up a water bottle cage. It can be hung from the top tube with the provided velcro straps.
I use the horn infrequently, usually to warn motorists backing out of driveways into my path, or opening a car door into my lane. Of course, if things are happening too fast, use your brakes or swerve, if possible. I would never use the AirZounds to signal a pedestrian; it is too loud. A bell is nicer, and a friendly shout is quicker.