I've been using Trillian for 2 or 3 weeks now and I find it a mixed blessing.
It avoids the neccessity of having multiple IMs running simultaniously but, unlike ICQ, you can only transfer one file at a time. The only way around that would be to zip multiple files into one file and send that. I'm not sure if there is a limit to the size of the file that you can send. There doesn't seem to be any maximum size in ICQ.
On average, though, I find it works relatively well.