At the company I work at we decided against IRC because of possible difficulties for non-technical employees. Currently we're using XMPP but we're considering other solutions.
The big advantage I see to IRC is how long it's been around. Plenty of clients and setups catering to IRC. And it's not going away anytime soon. Perhaps it's getting a little long in the tooth, as posed by moe at http://news.ycombinator.com/item?id=2069810