I have been getting tons of feedback on the new AIM Express we launched last month. The new application is really a great step forward for AIM. Built on top of Adobe Flash and our ActionScript3 library, AIM Express gives you a rich messaging experience without having to download any software. Since it is web base, you can use it on any supported browser (IE 6.0+, Firefox 2.0+, Safari 3.0+) with Flash 9.0. The best part is when we want to upgrade the application, we can do it without popping annoying upgrade dialogs, etc.
If you are a developer, you can see the ActionScript library we used to build the application here.
If you just want to check out the client, it is here.