I've used AirZounds for years. I use the plural because they tend to wear out - build quality isn't that great. I've just had to repair one which has under a year's use. It is, however, the loudest, most effective way of getting attention on the road. Shame you have to keep pumping them up!
Personally I don't use them on pedestrians as I discovered quite early on that you tend to get a "rabbit-in-the-headlights" effect; the peds, quite understandably, tend to freeze on the spot. A verbal alert is certainly preferable in such situations and, if done right, can appear not unfriendly.
Motorists are, of course, a different matter. I find that they tend to respond a bit better to the sound of a horn than to a shout. I've had more aggressive responses to shouts than to the horn but, I admit, I have quite a loud shout.
I find the AZ great for alerting, say, the motorist who is about to pull out in front of you because they haven't seen you. Like
BentMikey I do use it to convey my displeasure after a stupidly close overtake. Who knows, perhaps the overtaker concerned may learn something........
All things considered, it's highly recommended.