I guy at work has these on his dog's and reckons they are great.
If you loose your dog, you can get a location for roughly where the dog is as long as someone nearby is running the app. If you're dog walks far enough eventually he/she will pass someone.
From the website: "Every phone running the Tile app can help another person locate their lost keys or other tiled item. And it's all anonymous and secure. It's all about the network."