Once you install the plug-in, to start a video chat, just click on the "Video & more" menu at the bottom of your Gmail chat window, and choose "Start video chat." You'll have a few seconds to make sure you look presentable while it's ringing, and then you'll see and hear your friend live, right from within Gmail. You can click the "pop-out" icon to make the video larger, or click the fullscreen icon in the upper left-hand corner for a fully immersive experience.
Video isn't yet enabled in my account, but the Googlers say rollout is happening gradually over the next two days, to vanilla Gmail as well as Google Apps accounts, for Mac and Windows. What's your favorite webcam model and video chat service? How does Gmail's video chat stack up? Let us know in the comments.