It performs pretty much as advertised using it on my iPhone 4. You can make and receive calls over either a WiFi network, or over the cellular network. Even better, the app works with a variety of Android phones including the Droid, HTC Hero, Evo and Nexus One so you're not stuck with just iPhone users.
When you get the app you register for a free account. Your name and email address is all that's needed, and the Tango contact list will instantly display friends who have signed up. Contacting them is as easy as clicking on their name. You have the choice to do an audio-only call or a video call. Both iPhone 4 cameras are supported, and just like FaceTime, you can switch cameras while a call is on progress.
How well does it work? I tried a call to an Android phone running Tango and it worked fine. An acid test was a call from a friend visiting China. He called me from the Great Wall, and the video was passable and audio was fine. He said he was in an area with intermittent 3G and Edge, and his connection kept switching, but given the conditions it did work well. If the Tango app notices network issues, it puts up a warning on screen. Eventually the video locked up and the call dropped, but it was better than expected under challenging international conditions.
One note: when a call comes in you have to select a video chat or audio only. If you do nothing your screen will remain blank. Tap the Tango icon and you are up and running.
Getting Tango is a no-brainer if you want to video chat when there is no WiFi. It does not work on the iPod touch, but will work on the
iPhone 3G and 3GS, but there is, of course, no front facing camera. It is great for showing someone where you are, even though it's not practical to chat face to face.
This is the feature FaceTime should have had. Maybe it will come eventually, but until then Tango does a great job if you have good connectivity. I'd love to see a version that runs on my desktop or laptop, and I'd like to be able to record the calls. That would make for a pretty complete communications solution.