We previously wrote about a test experience with FaceTime where we called Apple's 1-888-FACETIME line. In that scenario we had some relatively stable conditions.
Last night I was at a tech-oriented event and we decided to try to push the boundaries of FaceTime. Using an HTC EVO 4G on Sprint's 3G network (4G isn't available in Rhode Island), we tethered the iPhone 4 to the EVO using its hotspot functionality, and then dialed up a couple of people using FaceTime.
As you can see from the videos below (shot with an iPhone 3GS), the quality of the calls was spot on. In one case the receiver was at an Apple Store, and in the other case the user was at home on his WiFi.
If AT&T's 3G can work as well as Sprint's did, FaceTime should have no issues moving off of its WiFi-only requirement in the near future.
UPDATE: The folks over at MobileJaw connected their iPhone 4 to the Nexus One and had a great FaceTime experience, too.