I don't use very many Instant Messaging pieces of software anymore. In my time, I have used Yahoo messenger, ICQ, Live Messenger (MSN), AIM (AOL), Gaim, Office Communicator (Microsoft corporate) and Google Talk.
I used to really like Yahoo messenger back in the day. I installed it recently and uninstalled it 5 minutes later. The reason is that all IM clients seem to fall victim to the same condition: install-additional-junk-itis. AIM had this problem as well; additionally the interface to the chat client was too clunky. So much more additional advertising, flashy stuff... all unrelated to chat. Yahoo messenger was the same. I didn't want to install because I feared that it would be like that, and I was right.
But I needed a chat client. I installed it and made sure to unclick all the additional extra features. After install, I looked at my web browser and saw that it had a Yahoo toolbar installed. That irritated me; I hate installing extra toolbars on my web browsers. All I have are the address bar, the search bar (with Bing as my default -- I switched from Google) and the Favorites bar. I value my screen real estate. I made the immediate decision to uninstall Yahoo Messenger simply because it installed the toolbar. Yep, I'm shallow. The fact that it had extra fluff cemented the decision.
AIM has extra fluff, Yahoo has extra fluff, and even Live Messenger has some extra fluff. You know what doesn't? Google Talk. It's a very lightweight chat client. It's a competitor's product but I prefer it precisely because it is streamlined and doesn't contain all the additional extra junk that I can't stand in a chat client. It just does... chat. And it lets me know when I have new mail, that's also handy I suppose. But it's unnecessary because I POP my mail every 10 minutes so why do I need to know when I have new mail?
So yes, I admit that I use Google Talk. But I only use it because it is so "thin." I like it that way.
PS - Office Communicator is my favorite chat client, but it works best within the corporate environment. It syncs with Exchange and tells me when people are in meetings, when they are away, when they are typing a message, and so forth. And no toolbars are installed.