My friend who lives far away has an iPhone. I have a mac, we use it all the time. I actually hate using Skype because some clown has my username. We use Skype at work, but I never get entrenched in a service if I don't get my username. Just a singular of data for you.

I haven't met in the flesh anyone who uses it regularly, and I know a lot of iPhone 4 owners. I'm sure there are people who do. It would be interesting to get some real data. My impression is, as with every video chat service for 30 years, it has very limited use because people just don't want to video chat most of the time.