Find My Friends works like this: You've invited a bunch of colleagues to an off-site meeting and people are running late. Instead of having to call or text each of them to check in, you open the FMF iOS app and see them represented as dots on a map.
But that is the extent of Find My Friends. You can locate people on a map, but can't tell if they're suddenly caught in traffic, or if they stopped along the way. You have to estimate their arrival time based on a dot on a map. To see their progress you have to manually touch a refresh icon on the screen. (Apple has promised enhancements to FMF in iOS 6 including location-based alerts, a.k.a. geofencing, and push notifications).
Enter Twist, a phenomenal iOS app that takes location tracking to a whole new level.
Instead of forcing your opt-in followers to constantly refresh a map to see your location Twist automatically lets them know when you're going to arrive via push notification. That way, if you make an unscheduled stop, or get caught in traffic, the person/people that you're meeting are automatically updated with your new arrival time. And it's the automated part that makes Twist so cool. You don't have to call or text them when you're going to be early or late, Twist takes care of it for you.
(Tinfoil hat types need not worry, both FMF and Twist are completely opt-in.)
While the iOS app provides the best Twist experience, other users don't need the app to stay in the loop. You can add contacts by their mobile number and they'll receive updates by SMS.
Twist is especially useful for working parents that shuttle kids to and fro. Instead of constantly texting your spouse "got the kids, be home at 6" you simply create a Twist that does the communicating for you. Twist is an amazingly powerful app and has earned a prime location on my iPhone's home screen.