I've been using it pretty much exclusively on my Mac for months (found out about it on Leo's podcast "Inside the Net"), so I don't have to clutter up the hard drive with installing instant messenger programs. One feature I hope they figure out how to integrate is sending and receiving files.
All in all, I'm very happy with Meebo.
Yeah this is really awesome everybody: Meebo, the online IM client. It works with Yahoo!, MSN, Google, AIM, Jabber, and ICQ. It's at meebo.com, and it's still in alpha, but I've been using it for over a month now and had absolutely no problems! Check it out!