I used Trillian heavily when I was on Windows. I give it the thumbs up. For Mac users, Proteus and Fire do the same thing. I recommend Proteus although it's still in alpha.

I used Trillian last year and it was very useful. You get all your IMs from one window. You don't have to have multiple windows open. Though I haven't used it in the last few months, it used to have problems with AIM every now and then and you wouldnt be able to sign on with Trillian. Until Trillian found a way to go around the new ban. It was a never ending battle. But it was well worth it. My 2cents.

I found the interface on Trillian a little too heavy looking. I am currently enjoying ICQ lite and think applications will begin to move in this direction in terms of providing the just basic features users require.

I use Trillian on a regular basis. Yes, there has been some problems with connecting to AIM or Yahoo!'s IM... but those are quickly taken care of with a patch... but it's not that often that you have to patch it... to me it's worth it.

Trillian is the only IM tool i use now. I even use it for IRC! love the tool and the team that's working on it.