It’s OK. The biggest challenge at this point is that it only works with GMail accounts and most of the people I chat with regularly don’t have one. Oh, well.
In terms of usability, one of the first things I do with new programs such as Google Talk (or the recent Google Sidebar) is to figure out how well it works in a keyboard/mouseless mode and how I might integrate ink into it. Sidebar is fairly straightforward. Google Talk isn’t.
The chat part of the program is decidely text biased–especially the one-line text window that grows as you type. A pop-up ink window would work. A separate ink client is another. Having a good developer story would be the one thing that would encourage me the most to switch from Skype.
In terms of Voice quality. It seems good. Keep in mind I only checked the app out for a couple minutes.