Apparently I'm the first one to post. Whee.
I use Microsoft Messenger for Mac, actually (it leaves a lot to be desired, but it's better than all of the other OS X MSN clients, IMO), and AIM for OS X (it's ugly, and looks straight out of Mac OS 9, but it works).
I prefer MSN over all other IM protocols, for various reasons. I.E. AIM still hasn't really implemented display pictures the way I'd like it to, and YIM is just.. well I don't care for it.
-John