I use it, along with AOL Instant Messenger. I'd say the Yahoo one is the better of the two, but more people seem to use the AIM. They're all pretty good these days, though, and MSN messenger is the other big one. You can even download programs that will communicate with all of them at the same time! They've come a long way from 10 years ago when ICQ (which I don't know if its even used anymore) was really the only one.
I use a program called Trillian. It communicated through AIM, Yahoo, ICQ, MSN, and IRC. Pretty darned handy. It seems like anyone I talk to online uses AIM so I don't get much use from the others, though occasionally I do go on IRC. I don't see much point in typing stuff to people online when I can just make a phone call, but it seems like everyone else is all into this text stuff online so I just go along with it.
Anyhow, it's convenient to have one wlel designed and straightforward program to handle all my internet chatting stuff instead of running a bunch of different programs in the background while I use the computer.