I had a chance to see FaceTime in person at this year's WWDC, and it's pretty sweet! Since it works only over WiFi connection, there's no worry about finding a strong enough 3G signal. Plus, since you're not using the 3G network, there's no worry about draining your precious data usage.
While this service is super-helpful for anyone who wants a quick demo or even more advanced demonstration, it seems a little Chatroulette-y to me! Have any of you tried 1-888-FACETIME yet?