Yes I've got a couple of the Loc8tor tags (the old style ones, not the new GPS ones). One of my old cats started getting a bit senile and going wandering. I used to go round the streets with my locating device following the beeps (louder for nearer, softer for further away - like that children's game where you'd say 'hotter, colder' until you found whatever it was) until I'd track her down in someone's front garden under a bush, or under a car or whatever.
I also use them when I have a cat that seems really ill in case it decides it wants to go and hide somewhere (as cats tend to do when they think they're dying).
So yes they're great. And the new GPS ones are even better of course.
The loc8tor tags look a little big for my cats; one of them is quite small (runt of the litter) and they only just tolerate their tiny little name & address tags. Been reading on another forum that there's a company that is designing locator tags for birds (Marshall telemetry), so I'm going to try and find out a bit more about about them.