I really don't care for AIM. I run an on-line radio station and take requests from listeners, and it's widely used, so that's why I have it.
I tried google IM, didn't like it either, but I hate learning new things because it takes up valuable RAM in my head.
Hate MSN messenger - it pops up - I HATE things that pop up at me - it reminds me of a very old lame horror movie called "the boogens".
Don't want trillian, don't like being "managed"...
ICQ is OK but seems to suck up too many system resources...
So yep, I'm all about yahoo IM. Maybe I just love the smileys.
Oh, and this spell check feature rocks!