Sure, there is AIM for many platforms, but when I first got my MacBook Pro, AIM for Mac would crash and relaunch in a seemingly infinite loop. I understand that it has been in a beta state since 2004 until recently.
Adium already does what this official AIM client for Mac does; don’t bother downloading it. Plus, you can connect to Yahoo!, Windows Live, and many other services with Adium.
The only issues with multi-protocol IM apps like Adium is the unreliability of file transfers and a lack of webcam support. AIM file transfers over Adium are fickle at best. Yahoo file transfers work well and with excellent speed. MSN transfers… Don’t even bother! I barely get 1 kbps over MSN.
With all that said, I use Adium for all my general chatting. If I want to use my webcam, I load up the official Yahoo! client, and if I have to send a file over MSN, I load up the official Microsoft Messenger. Of course, users can always upload files to MediaFire or ZShare. The Yahoo, AIM, and Microsoft IM apps for Mac OS X are lacking a lot of features their Windows counterparts boast. Adium works just fine for general purposes.