Google Talk uses the Jabber protocol. Since they opened up their network to link to any other Jabber server I can send messages between my Google Talk account and my main Jabber account. I've not actually used Google's client. I use Psi on Windows and Kopete on Linux. I now have another option for times when those are not available.
I don't have many contacts who use Jabber, but there are ways to use it to connect to those on MSN, Yahoo etc. You just lose the useless things like sending pictures, sounds etc. I ought to try linking to MSN from Google some time. One of my reasons for using Jabber is to support open standards. I use Ogg Vorbis for my music for similar reason. You just have to accept that most of the world is still using proprietary systems.
If anyone wants to play with Gmail or Google Talk, then drop me a mail. I have 100 available invitations!