I've got about a dozen invitations to the Windows Live Messenger Beta. If you're interested in trying it out, leave a comment on this post with your email address (if you already use MSN Messenger, leave the address you use to sign in to it).
I've been using all four of the major IM clients for awhile now, and have to say that the new Live Messenger is definitely at the top of my list. I'd ditch AOL and Yahoo in a flash if more folks would move over. Google Talk I don't mind, just because it's clean, quiet, and usable (but unfortunately lacks in features).
So, if you're interested in trying it out, leave a comment (and then feel free to post your reviews later -- the Messenger team has a blog, and is always looking for ways to improve -- in fact I recently sent over a list of 10 suggestions based on my beta experience).