I've watched it grow from just a very simple IM only tool to a clean IM tool with only whats required. eg. IM, file sharing and VOIP.
I used the VOIP for the first time last night talking to Andy Allan after the UG meeting. I have to say I'm very impressed. The codec that it uses are absolutely crystal clear.
The GoogleTalk beta was only open to those of us with GMail accounts (does anyone not have a google email account these days?), but now its open for everyone to use.