I use AIM only because all my friends use it, and MSN because all the people on the forums I go to (including here) use it, and Yahoo! IM for my Halo PC clan since it's connected with Yahoo! Mail and is easier to get clan updates and news.
I like GoogleTalk's user interface though...very clean and simple. I never liked ICQ, nor got how to use it.
Yeah, definately! AIM is very widely used here in America, but all my cousins and forum members that live in Canada or the United Kingdom (or some other country that I failed to specify) use MSN. It's kinda weird...