Use IM for non-committed conversations only

I read Scott Hanselman's post on communication, and was reminded of all the times when I had to kick myself and use phone over IM - I just couldn't stand how slow the conversation was going.
In terms of effectiveness, IM is good to have when you are having a non-committed conversation, meaning you are doing several things while chatting.
When you need to dedicate your full attention to the conversation, use phone - it's faster.
Not only most people are able to speak faster than they can type, IM forces you to talk serially - while talking is parallel by it's nature.