I use both Fire and iChat. Fire for Yahoo and one AIM identity and iChat for the other AIM identity. I could use Fire for my ICQ id, but I use it so infrequently that it's just as easy to launch the ICQ app when I feel like it.
I prefer to have one chat window per person. THAT way, I know when someone wants to converse, or leaves, without having to sift through drawers. Also, since I'm on the 15.2" TiBook, there is plenty of screen space for several windows, if I choose.
Fire, also, has provisions for ALL the chat types, so you can use it for everything if you want to. There was a version or two that had issues when ICQ updated/changed some of their server side stuff, but that was resolved with an update and the issue hasn't returned.