IMing is something my city already uses for staff. It's really a great tool to have when you need to tell someone something that is more urgent than an email, but not urgent enough for a phone call or a face-to-face meeting.
I also used AIM a lot when I was in college. IM was the best way to have a conversation with your friend in another dorm or room without having your roommate overhear everything. ;)
Now that I'm out of college, the only place I truly use IM frequently is at work. I'm hopeful that one day we can maybe explore IM Reference. It seems a lot more of our patrons would be comfortable chatting online instead of face-to-face. I'll just have to wait and see!