I will agree with Drakino that Trillian is a great IM client. However, I have one major warning for you:

AOL keeps finding ways to block Trillian clients. Their last round of ECMs had the Trillian developers scrambling to fix it, and we were offline for a while. Things are stable now, but I don't know how long they will continue to be that way.

So if you only have AIM buddies, and don't need to be on more than one IM network at the same time, then don't bother using Trillian, stick with AIM.

But if you want to be on all of these IM systems all at the same time, then by all means, use Trillian, it rocks. I personally use it for ICQ (primary usage), one or two AIM buddies (secondary usage), and once in a while, IRC. It's really great having all of that in the same user interface.