I watch it in action around our shop. It's quick, easy and damned intrusive. That's why I don't like it and won't turn it on. Much of my work requires thinking and writing. I found that I do neither when a box pops up on the screen with an invitation to chat with someone down the hall.
Instant messaging is great when one is coordinating with another or in meetings.
It's not so great when one has to do the hard labor of figuring out things. Instant messaging fits right along with cell phones and other gadgets that keep people in touch 24 hours a day.