I must agree with Toby on this one. I've found limited benefit from internal IM. The payoff seems too small in comparison to the cost of implementation and ongoing operation. In my experience, the only useful application of IM within the same building is in the context of a larger collaboration package (where IM is a component) such as Webex, used for synchronous e-learning or e-meetings. IM can be used by participates to communicate with each other, or the presenter, in a less "invasive" manner. In this case, however, you're paying for, and managing, a larger suite of collaboration tools in which IM is already included.