I've just downloaded a copy to give it a test run. Google Talk has an uncluttered, no-nonsense interface that is quite simple to use. The application integrates with Gmail, and can notify you when new messages arrive. A summary of new messages is displayed in a small pop-up and you can click to go directly to your Gmail inbox.
I haven't tested out the IM or Talk features as yet, but I’ll run it through its paces directly
Here's what Google has to say about Google Talk:
Google Talk is a downloadable Windows application that enables users to quickly and easily talk or send instant messages to their friends for free. Calls are made through your computer using the latest voice technology; all you need is an Internet connection, a microphone and a speaker. If you don't have a speaker or built-in microphone, you may want to consider buying a headset, which will also provide the best voice quality when using Google Talk.