I have found IM to be incredibly useful. It's great for seeing if a coworker is actually at their desk (love the status indicators that turn yellow when a computer is locked or idle), asking a quick question, having a private conversation (I'm in the middle of cubeville with multiple people per cube, lowish walls and no doors) and getting the attention of my cubemate when he's got his headphones on - without having to yell or throw a squishy ball at him.
Unfortunately (or maybe fortunately, productivity-wise), I only have IM on the company's computer network. There are many times when I'm trying to coordinate something with someone outside of work where IM would be the best way to do it. Since facebook chat doesn't work either, we usually end up shooting lots of quick emails back and forth. It works but isn't near as efficient.























