Anytime I hear the name "America Online" I just run a mile. I vaguely remember using their terrible mail client and it's banal "You've got mail!" voice warning and ever since then, I won't go near anything they've made.
America Online Instant Messenger for Mac (AIM for Mac) is a completely revamped version of their messenger client but predictably, it doesn't really offer anything that other messengers can't. Once again, it puts a high emphasis on sounds allowing you to configure a new sound for every buddy when they come online or contact you. You can also add a huge number of AIM Plug-ins including games, stock updates, weather forecasts etc. This plug-in feature is possibly the only thing that distinguishes it from the pack, making it easy to expand your messenger client by clicking on the magnifying glass at on the bottom bar.
Other standard features are included such as video chats, phone calls and AOL email integration. Although the service is free, you have to sign up for an AOL account. Text messages also cost extra and are only available in the US at the moment. You can edit your buddy icon to give yourself a unique image and also assign yourself a sound that others will hear when you sign on.
AIM for Mac could be a good option for those that love to customize their messenger client with sounds but unless you're an AOL mail user, there are better alternatives, such as Adium, out there.