Hello, my name is AOL Time Warner, and I now own all content you send through AIM and can modify it if I please.
Therefore, I ask all my chat partners to download Gaim, and gaim-encryption to protect our mutual intellectual property, as well as the sanctity of our communications.
To you Trillian folks, I used to be among you, but the encryption scheme is weak, and is not cross-platform. The vanilla AIM client supports encryption, but after hearing this, would you really trust AOL?
Use a Mac? I don't care, Gaim works on it, use it. Windows support for Gaim is delightful. And if you run Linux, you probably already are using Gaim.