We're a very distributed company, with some projects being worked on by people in a dozen locations, so some form of IM is needed.

I'm a big XMPP proponent (and Voxeo acquired my xmpp-based company) and we do use XMPP for a lot of things inside Voxeo (like Phono, our jquery softphone: http://phono.com/).

Every voxeo employee has an XMPP ID, but they don't tend to get used. People gravitate to Skype naturally. It's the user experience on the client side of XMPP that keeps people from using it extensively for internal communications. At any given time I've got 40+ group chats going on and we create and destroy group chats many times a week for specific needs. The ability to make a voice call to one or more participants of the chat is also a winner.

The fact that everyone on the planet has a Skype ID also helps. We can easily pull partners and customers into chats as needed. They're already on Skype, so we don't need to teach them anything new.

It's a lesson in usability, certainly. Create a client that allows users, without any assistance, to create groups, move seamlessly between text, voice, and video, and has a foolproof signup and setup process, and folks will use it.