It is totally opt-in, you have to authorize people to be able to track you, and for how long. It was available previously too, but now Apple is alerting you to the availability of it as a free download.

I've used it with a friend. He was driving the subject of a surprise party, and I was able to track him surreptitiously so as to get the timing of the surprise right for the subject. It was highly effective.

I also see it being useful in group outings where you might want to meet up with people. Picture three or four friends going out to a theme park or county fair, all perhaps going to different rides and attractions, but wanting to be able to easily see where the rest of the party is, and make it easy to find people when it is time to eat together or gather up and head out to the car.

Totally opt-in, and under your control the entire time.