It works very well but you MUST have a G5 processor, the faster the better, to have multi-user video conferences.
Obviously it works most effectively when all the users are running iChat AV 3 on G5's
Quote from Apple
"To join a multiperson video conference, you need any G5 computer, or a 1 GHz single processor G4, or a dual processor 800 MHz G4, and a broadband connection to the Internet.

To host a multiperson video conference, you need any G5 computer, or a 1 GHz dual processor G4. In addition, you need an Internet connection that supports a minimum download bandwidth of 384 Kbps, and a minimum upload bandwidth of 384 Kbps."

You can find more information about it's capabilities at the Apple site http://www.apple.com/support/ichat/

Hope this helps

P