To be fair to both of them, prior to Trillian Astra (Version 4) it was really rather sub-par past about 2005-6, and Trillian Astra took forever to come out. I know, I was one of the closed-beta testers, and for at least 3 years people would ask me about the chat program I was using and I would have to tell them “you can’t have it.” Finally, they released Astra (Trillian 4), and relatively rapidly followed it up with v5.
Version 5 has one of the “killer apps” of IM, something that I didn’t know I couldn’t live without until I had it – what they call “continuous client.” Seamless integration across multiple client sessions or multiple host devices; with associated chat history sync &c. Yes, this does mean their servers can conceivably sniff your traffic – the options can be turned off. OTOH, it means you can keep chat history without storing it locally, which is a nice feature if you don’t fully control the machine (such as a work-owned machine. Check your IT policies concerning third-party programs and web services before using such).
Now, since Alan mentioned he used Adium, I’m going to deduce he’s a Macophile :) I don’t have any direct experience with either the OSX or iOS versions, but if they are anything like the Windows and Android versions, they’re pretty good.
Check it out – the basic version is free (ad-supported) and has all the features but cloud storage of chat history.