I bought this just to play with the vibration detector. Then I decided to annoy a cat that was scratching where it shouldn't. I set it off about 20 times while testing it. Once it settles in, the slightest jolt will set it off. It reminds me of the old game 'Operation' - see how far you can move it without triggering it.
The siren is not only loud, but an ascending sine wave that hurts the ears in less than a second. It shuts off after about 20 seconds, but believe me, you can't stand it that long.
Back to the cat story: I rigged it up with thread to scare the cat when it went near that spot. Trouble is, I go there too. After accidentally tripping it about 6 times, I got mad and kicked the hell out of it. It flew across the room, bounced off the dresser, battery and battery cover flying off. Still works fine with the same battery.
It's not waterproof. It might be OK outside in a ziploc bag. I'm thinking of testing it on the resident deer population.
No question, if you need something to attach to a door or window (or several), this is a cheap solution. A string or anything to knock it over or make it wobble is all you need.
It 'could' work as a personal alarm, but I don't recommend it. After you turn it on, it takes 15 seconds before it's armed. That's a long time if someone is attacking you. On the other hand, if you want to let some stranger knocking at the door know just how you feel about him, that 15 seconds while he's introducing himself might be perfect timing.