I've been experimenting with low-cost, DIY home security solutions recently. This product, so far, looks promising.An initial test indicates that all four units work as advertised. I've read some reviews suggesting that they fail after a few weeks or months. I'll test mine in time and, if appropriate, provide a 5 (or 1) star overall rating.The installation can be a little challenging. You'll want to experiment with the distance between the magnet and the alarm housing. Too close and the friction might interfere with the alarm trigger and similarly if the two parts are too far.My application was for home security so I sought to create an installation housing so that special tools would be necessary to remove or destroy the alarm. Specifically, I used two sheet metal screws positioned so that they were underneath the alarm. I wrapped a length of malleable, solid wire around the top 1/8 of the screws (not tightened flush), cut a 1/2" square of foam rubber to cover the top of the alarm, then pulled the wire so that it was wrapped around the other metal screw. I used some electric tape to cover the alarm switch.The first one was a little time consuming but once you've done it a time or two it is easy. Of course, if the thief is familiar with the device he/she could simply remove the electrical tape and turn off the alarm. I also had to use some clear packing tape and a little super glue to position a magnet on one install to make it work.Hope this helps.