While I personally don't use Google Talk, it does look good and easy to use, just like a few IM applications out there.
Here's a snippet from Google Mobile Blog:
We've just released in the US a new version of Google Talk designed specifically for the iPhone and iPod Touch browsers. In addition to sending your friends Gmail messages from your iPhone, you can now chat with them while you're on the move, too! In your iPhone browser, just go to www.google.com/talk, sign in and start chatting. That's it. Google Talk runs entirely in the browser so there's no need to download or install anything.
Point your iPhone browser to http://www.google.com/talk to download it
[Via: Google Mobile Blog]