MY experience with this product so far is very positive. I have the sensor attached to my cat's harness, and she handles the weight with no problem (she's a little over 9 pounds). One think to note is that when your pet is within range of your home wifi, there is no location tracking function, even if outside your specified safe zone. The app just shows the pet is located at "home". Your cell phone location is always shown, as it's provided by your phone's gps. Once your pet is outside wifi range, you get a text alert, and a button appears to start tracking its location. If you select tracking, the light on the unit will start flashing, which I can see being a big help in locating your pet in the dark. It does take a few minutes (maybe 5 minutes or so) after leaving wifi range for an alert to arrive, which I wish was a bit quicker. But once tracking starts, it updates frequently (I have it set to update every 3 minutes I believe). The positions provided on the app seem to be accurate. Overall, the tracking works well, the app is nicely designed, and there are lots of additional features on the app, primarily related to your pet's activity level. When at "home", activity is tracked via wifi, which saves most of the battery for "emergency" tracking when needed, which is a very clever design. The Whistle is a great improvement over previous gps tracking devices I've used with my cat.