I still use Trillian... I go back and forth between using the 3 "real" clients (Live, Yahoo, AIM) and Trillian.
I get sick of Trillan's suckiness (locks the whole damned computer up when it starts) and lack of compatibility with all the features, but at the same time: uses less resources than 3 separate ones.
I'm personally holding out for Astra- but that seems to be taking forever!
Just a little note: When attempting to install this new version of Yahoo IM using 64-bit Vista Ultimate, the installed was canceled due to incompatibility with 64-bit. Not sure if Yahoo offers an x64 version.