This is going to sound more creepy than it is, so bear with me.
When Apple introduced Find My iPhone, I thought it was great for finding lost technology investments. I used it several times to confirm that, yes, the iPhone was indeed lost.
When Apple first introduced Find My Friends for iOS 5, I thought it was cute. It seemed like it would be very useful to people with active social lives and lots of friends. People other than me. But since then it's become one of the apps I depend on regularly and I would truly miss it if it went away.
My wife is in night school, which means she is very busy and I am very worried. She is often too tired to text me before she leaves school for the commute home. Find My Friends solves that problem for me. A quick check-in shows me precisely where she is. It is a fantastic peace of mind. It's also great for planning family meals. Rather than calling her to see when she will be home, I just check Find My Friends. When she's 10 minutes away, I quickly dab my forehead with water and pretend I've been slaving away for hours. Priceless.
I also use the service with my mother, living 3000 miles away. When she has a long trip, I can check in to make sure her travels are proceeding as expected. It's not creepy. It's convenient for both of us. She doesn't need to stop and text me and I don't need to worry. With both of these cases, the service works flawlessly. I've explained the system to my wife and mother, and they both know how to revoke my privileges. It's probably the most considerate tracking system that could be designed.