Yes, go AOL haters!!!
As for me... I use a very, very hacked version of MSN! Tried trillian but couldn't get used to it.
As for what's popular - AOL pretty much started the whole instant messaging thing, so people started using it heavily before they did the others. A lot of it has to do with what your friends have. You're going to use whatever IM you have friends on, and since AOL was able to get into that market a little earlier than the others, people built up their buddy lists around AIM. Then when MSN and Yahoo became more popular, there was no point to switching.