I have been using IM for a while. The advantage is that you are carrying a conversation. As in a conversation you receive instant responds and comments. The disadvantage is that you need to online at the same time.

I use IM to communicate with a friend that lives in Spain. Calls are to expensive. Sometimes we used webcamera, and the phone option. My friend likes to use the call option since she doesn't like to type. My friend is in Puerto Rico on vacations and we still can use IM. The other day I was green with envy :) because she had been at the beach.

In a professional setting, I have seen libraries that have staff available to answer questions by using IM. If I was a patron, I'll use it.