I work at MS and we use Lync Communicator a lot. Group chat seems to work well (as far as I can tell). Then again, who knows what server they have us connected to - it's probably v.latest.

I've read that there is an XMPP federation service [1], although having never touched it I don't know if there are any caveats.

[1] http://ocsguy.com/2010/11/29/deploying-lync-for-xmpp/