I installed it this morning. Works well, of course your friends have to have cams and Gmail and be in Gchat (pretty much like with AIM or iChat), but many of my contacts don’t use Google’s features in this way.
The real advantage of this is simple: it comes from Google. Which usually means more reliability and common sense functionality. Also, it allows those using Gmail for their email to use one application instead of multiple.
One roadblock I encountered was even by detaching the chat, when I closed (not quit) my Safari browser my conversation died too. No doubt user error and lack of time getting used to it.