I have one installed at my house.
I have had it for nearly ten years now, same one too, monitored too. The alarm company moved it to my new house when I finished building it.
Never had a false alarm with it. I do pay now an extra $5/month since last summer so a private security firm will respond immediately instead of them phoning me first if it goes off.
If/when I am away on vacation, the last thing for the alarm company to do is try to get a hold of me first, because that won't do me any good.
I have never used it, but it helps knowing that it's there.
Heeter