R.M.
Depending on the service, it's pretty reliable. The tracking service I had with AT&T was not bad, but the "find my phone" tracking on the iPhones is awesome. You can locate the exact spot.
I love(d) being able to track my kids. Now that they are "grown," I mostly don't need to. But I've been known to do it a time or two since, if they are on a road trip out of state and I'm worried about them, for example.
I don't consider it "spying," and I think parents have every right to locate their teens. I did not hide the fact that I could locate them -- I made it mandatory. I see no reason why I should have to sit around and worry myself sick about my teens just so they can feel like their "privacy" isn't being invaded. That's just ridiculous, in my book.