Less controlling than a leash, which I swore I'd never use until my just-turned-2-year-old decided to play "daddy chase me" at an airport...more controlling than just giving the kid a phone so you can call them and ask them where they are.
But now I'm not so sure. My wife reminds me that I once advocated putting a chip under the scalp (less likely to be removed). Still, and perhaps especially since the DHS has been established, I'm not all too sure that having even our little ones with devices on them that can tell anyone where you are is a good idea.