The mac trillian is pretty ugly and history browsing is terrible, but I use it. I use it for one single reason: it's the only chat client I've found that can sync between multiple devices. I fire it up on my mac and on my android phone, and not only does it give me the same chats on both, but it's also really good at figuring at which device I'm at and sending notifications there (for example, if I'm working at my desk it does not notify me of new chats on the phone, only on the desktop).
To me this is the killer feature. I would, however, switch in a heartbeat if there was a more featured and attractive app that could do this.
I use it because I loved Trillian on Windows but Trillian on mac is still a buggy, alpha product.
One thing that's incredibly hard to get right with modern IM is intelligent multi-device routing. It's incredibly frustrating to have your phone buzzing next to you as you're having a conversation on your PC. Using Trillian on all my devices prevents that and that's reason enough since Adium doesn't have a iPhone app.
