I don't IM at work unless something is really wrong...like if I know one of my friends is upset, I'll look to see if he or she is online, and I'll try to chat with them in between work.
I do peek in here at work (obviously) and send e-mail, but if I was on IM at work, I'd have so many people IMing me that I'd never get anything done. The couple of times I've signed on from the office, I've always had to answer someone's IM with "Not now, I'm at work, catch up with you later"...which is why I tend to avoid it.