I've just tried Google Talk and I'd say it's better than MSN for instant messaging. It loads faster and it looks better.
It's also platform independent, which none of the others have done yet (why?). It says that it will be platform independent soon, but it's not yet. So if you don't have a Gmail invite to give away then Google Talk doesn't let you connect with non-Gmail users. Google says it's working on it.
And I'm yet to decide whether I like it from a VoIP perspective.
If you want to try Google Talk you need to be invited by an existing member. I have some invites so get in touch if you want one.
Check out Neville Hobson's report on sound quality of the VoIP calls and more detailed comparison with Skype.