We're using ejabberd at work as well. We're ~70 people and have got email address == xmpp handle, and we're using team-specific and general MUC chat rooms.

I do miss using IRC at this company, but at some point around 08 or 09 most people switched to Skype (as we'd used to for client communication anyway) and forsake IRC, but not all were happy with it. So in early 2011 (I think) we've had some polls and 3/4 preferred XMPP over Skype, so we switched. Personally I'm very happy with it.

My experiences with client libraries in PHP were horrible, though. xmppy (Python) and smack (Java) are much better.