hmmm lets see...i've used:
Adium: good but the support dropped off, and the one mode i was going to use the most was simply given up on it seems. doesn't support mac.com names
Fire.app: Another good alternative, but has always had same nagging issues. One interface option, and it hangs like crazy any time it runs into connection problems. did like the eye candy of flashing windows doesn't support mac.com names
AOL AIM: Piece of crap. Slow, buggy, not customizable in the right areas, too customizable in the wrong ones. overall clunky and unimpressive.
Proteus: Always has been a promising app, never has delivered. Basically dead in the water altho this guy goes on hacking sprees sometimes. never quite gets his services to work properly. doesn't support mac.com names
iChat: My pick. I played with it for about 10 whole minutes and I knew it was going to be the surprise app for me. It seems to be responsive, has build in Rendezvous support, has a clean interface that is in the mode of the iApps. You can turn the eye candy off. Integrated with Address Book and Mail. Icon to go along with Airport, Battery, etc. click and drag file X-Fer!! I was extremely impressed.
Anyways that was my $0.99