I generally use Gmail's internal chat program, because I like the archiving function, and, primarily, because I'm lazy and it has everyone I can chat with right there in the address book, and I don't need to open another window. Before that I used AOL instant messenger, and occasionally Yahoo's instant messenger. I don't chat all that often, and really chose my client based on who I needed to talk with and what they were using.
But today a friend offered to help me fix my pesky photo thumbnail problem. We were chatting via Gmail and he ran me through some diagnostics and we got nowhere. I said, I wish I could send you a screen shot, and he said, if you have Hello you can. I'd played with Hello before and thought it was cool but perhaps a novelty. But now I get it, just how useful it is. You can send your screen back and forth to people as well as looking at pictures together. You can see what they're looking at while you're chatting. It's a totally different tool, and a much better one, than a basic chat.
I recommend downloading it and playing with it. It's supercool.