I just tested out Facetime, the video calling technology built into the iPhone 4, with my friend Rob.
I've used a number of video calling technologies before (e.g. Skype) but this is the most natural by far. You immediately forget the technology; it just feels like a face to face conversation.
The only problem is that it's a closed system; you can only call other iPhone 4 users. Let's hope Apple open the technology up. Otherwise it will be the same old story; due to the network effect an inferior but open technology will come to dominate the market.