I have a boatload of these - probably 40 installed. Got most for either $5 or $9 each. They've mostly been unbelievably reliable... except for 2, which when the (non-rechargable) CR2 battery got low they started to throw bogus open/close events. This wouldn't be much of an issue, except that they are on windows I monitor with HSM, and this happened when I was away (thus it was armed-away). So all of a sudden I started getting HSM alerts for a open/close events on window on the 2nd floor, while I was 2000 miles away. Thankfully with remote admin I was able to just go in and remove that window from HSM. But then a few days later it happened to a different window.
This happened when both devices were at 25% battery, and they'd been in service for nearly 2.5 years. These were still on the original battery that came with the devices.
I've replaced about 30 batteries in these this year (since they are all about 2.5 years old), and most of them have just stopped reporting as the battery got low.
I still love these contact sensors.
