Well, you can't have everything Harry...
I've been using Trillian for almost a year I think. I love it because I know people on all four services. I used to use ICQ exclusively before I found out about Trillian, on which I had a ton of people. Unfortunately, it always seemed like there was a friend here and a friend there that used one of the other three services, and it was very frustrating. Everyone has their favorite and you couldn't get them to switch because they had family/friends that were only on that same service. Now it doesn't matter, because with Trillian
I have them all! Mwahahaha.
Also, another great feature about Trillian is that it also supports IRC, and you can use it to also connect to the Mustang Works chat room!
Port: 6667
Server: chat.mustangworks.com
Channel: #mustangworks
Furthermore, Trillian also has secured (encrypted) messaging between Traillian clients through either ICQ or MSN I believe. Make sure in your option to enable "Secure IM" and a lot of times it can use this. Notice the lock on the message window will indicate this. Also, Trillian is working on the next version of their client, and I believe it will support voice communications too. I'm personally also hoping they add video, like the native yahoo client supports, because I use a Web cam with a couple friends. Right now when we want to do that I disconnect from Yahoo on Trillian and run the actually Yahoo client to do it. It would be nice if I didn't have to do that.