I have my iPhone Jailbroken and must use the FaceTime feature over 3G networks because I can't connect to Wi-Fi on campus, and it is VERY usable. I have the unlimited data plan, so overages are a non-issue. I've conferences with my friend while he was on Wi-Fi and the video and audio was really really smooth.
I have also conferences with my mom who is back home (and she's also using the 3G connection) and the video and audio are great too. I've actually called her while she was driving home and that's when there was lag on her end because of the fact that she's moving and is probably jumping around to weak signals and different towers.
But FaceTime is definitely usable over 3G.