I've used it with 3 different people for a total of 5 times since launch.
It's neat, but as others have said, Facetime needs to be in more hands for it to really be useful.
My mom just upgraded her 3G two days ago, and I Facetimed with her for a couple of minutes while I was at a restaurant waiting for my food so we could test it out. I'll be going back to college in a couple of weeks, so it'll be nice to use to see my family back home face-to-face.
I've iChatted with my parents several times while away in College, but this is a lot more convenient because you don't have to hassle with getting on the computer which my parents don't use much. What I like about the iPhone is that since it's so simple and handy to hold, you can Facetime in occasions when you just want to have a quick word with someone.