Have been using trillian for some time now and some of the features are really cool
Pros:
Tabbed chat windows are awesome. Its possible to attach chat windows to different parent windows and the chat sessions run as tabs within the parent window. This is simply awesome while talking to multiple people. The screen doesnt get cluttered at all
Window transparency - This is a really cool feature to set transparency levels of the chat windows so that even with text being typed, one can see whats going on behind the window. Really cool when you are chatting while watching a full screen movie, or working with any other full screen application for that matter
Amazing collection of emoticons. Although not all of these emoticons will appear as bitmaps if the user at the other end isnt using Trillian, between trillian users, the graphics can be far more expressive
Archiving of chat sessions as simple text files, which make it that much more easier to manage (ofcourse, some people might call it a security risk, but then for me its simpler this way as no one else uses my system other than me)
The coolest feature of all (nothing unique though) is th eability to connect to multiple service providers, so you can chat to all your friends on diverse services from within one single client.
Cons:
Audio chat is not integrated seamlessly and uses the quicktime plugin which is not my favourite way to transmit voice
Video chat doesnt exist (but hey, Google didnt put it in theirs too !!!!)
Conferencing is a tricky proposition at best
Overall, if it is just chat you are looking for, you should definitely consider Trillian. An extremely simple to use, stable (have been using it non-stop for around 2 months now .. 24x7, hasnt crashed even once), very intuitive, and very user friendly UI design. The developers of this piece of software have actually drawn use cases of usage and built the software to meet most of those use cases perfectly well.