I've used it a few times (for international video calls, easier and more reliable than Skype). My anec-data is that many people don't know what it is or what it does. I've introduced it to about half a dozen iPhone 4 users.
There's also the anec-data for me that video chat is just meh. I'm about to jump onto a skype call with an international colleague - do I want him to see me in my t-shirt on the couch? No. Do I want to shave, suit up, and head to me home office? No. So voice call it is for me in a majority of situations - only I believe FaceTime is either video or nothing, with no voice-only option.
There's also the anec-data for me that video chat is just meh. I'm about to jump onto a skype call with an international colleague - do I want him to see me in my t-shirt on the couch? No. Do I want to shave, suit up, and head to me home office? No. So voice call it is for me in a majority of situations - only I believe FaceTime is either video or nothing, with no voice-only option.