I'm using Trillian... and I think it's much better than any other IM client out there, though I'll be the first to say it's far from perfect.
It's user interface (in the Trillian Control Panel) has become WAY too dumbed down... but that doesn't bother me *too* much, since I rarely need to use the control panel. It does use a bit too many resources for a single program... but definitely less than the 4 programs it replaces (AIM, ICQ, MSN, Yahoo). I also greatly prefer the nested grouping that Trillian allows, the metagrouping, etc. It logs everything in both TXT and XML format... with the only good way to browse the logs being via notepad. They have a log browser that parses the XML file and presents it that way... but I personally hate the way the log viewer works. That said, I don't know how I'd improve it... which is why I haven't written another one.
Yeah, it has flaws here and there, but overall I think it's the best one out there. I've tried the 'factory' installs of each of the 4 clients, I hate them... I've tried hacked installs of AIM... hated it... I've tried GAIM... begged to have Trillian back... :-
PM/IM me if you want to know anything specific that you can't find on the Trillian website.