IM has to be one of the most interesting forms of communication. You get to "speak" with someone almost like you are on the phone with them, with the major distinction that more often than not you don't plan to IM someone.
They just show up online and you begin your conversation. In this way IM acts more like a chance meeting at a store or something.

While, I prefer the immedacy of IM over email, I also like the asychronize abilities of email. Email is more practical because you know your message is going to be received. On the other hand, I was talking with my younger brother- and sister-in-law, neither of them have email addresses because they see all their friends through IM. They just leave away messages for each other in a pseudo-email fashion when they can't chat.

A very interesting culture has sprung up around this. One that I am only tangentally involved in, and think that I will stay on the sidelines.