Gtalk is my favorite IM client. I simply love its clean, light and unclutted interface. One complaint I really have from the folks at google is their constant refusal to release a linux version of their client. They want us to use variants like Pidgin( previously, Gaim) and Kopete, but none of these clients support voice and file transfer that the gtalk desktop client for windows offers.
Gmail's gChat is a killer tool for people who either don't have the gTalk desktop client or hate to use Pidgin or Kopete on linux.
Recently, I found the invisible mode in the status and it works wonderfully well. It still doesn't feature in the desktop client so not many of your friends wouldn't know you are secretly snooping on them staying invisibe ;)
Here is the screenshot...