Since I dropkicked PCs out of my household five years ago, I've switched between iChat and the wonderful Adium. Trillian has a lot to live up to when it comes to instant messaging on the Mac. Adium, to start with, is free. Trillian costs Windows users $25USD to get full access to all of its features (the Alpha version for Mac is free). As stated when the original alpha was released, Trillian is rebuilding its software for OSX from the ground up.
I downloaded a copy of the alpha to try, and I am not impressed. While Cerulean Studios did warn that the alpha version of Trillian is lacking features, the entire experience was just unpleasant. No buddy icons from any of my IM accounts came over and the program looks nothing like the picture included with this article (taken from Cerulean Studio's blog announcement.) Having one-button clicks to access your different accounts is nice -- when it actually works. The most glaring of the missing features is no group chat support, which will be a turnoff for many. Other missing features include a lack of voice and video support, chat logs and more. You have to sign up for an Astra account to even start using the IM client.
When I was on Windows, the free version of Trillian was my IM client of choice. The Mac version has a lot to live up to, and maybe it could hold its own against its Windows big brother one day. But for now, I'd recommend sticking with much better, and free, alternatives such as Adium and Pidgin.