The timing on this couldn't have been better. Getting married next week with lots of out of town family and friends coming, most of whom have iPhones. I'll just setup a temporary event for the week so everyone can help each other out. "What? You need a ride to the rehearsal? I see so-and-so is near you, I'll just conference him in and see if he can pick you up." "You never got the address of the restaurant? No problem, we're there now. Just look us up and get directions from the Maps app." At the end of the week the event ends and everyone is back to being private again. That's way better than foursquare or facebook places, where all my friends and relatives, who don't know each other, and will probably never meet each other again, would have to all friend each other then remember to unfreind at the end of the week.
Edit to add: One good feature that I hadn't seen mentioned anywhere before is that your iCloud login is required every time you start the app. That is, unless you setup the phone's passcode lock in Settings. Nice balance between convenience and protection from strangers who might seal your friends' phone at a bar.