IRC is an indispensable part of my work. The team I'm on uses a private server for team discussions (works great when someone is working remotely), and we also are in a few public channels on freenode. On freenode, I'm in #openstack to talk about cloud object storage infrastructure, #eventlet to learn more about it, and #cloudfiles for Rackspace product-specific questions. #openstack has been pretty active in the last couple of weeks (since we released the code) and has been pretty good for helping people who are learning about the systems. Internally, I can't imagine not using IRC. If I choose to work remotely, or if I'm working odd hours, it becomes a very important tool to keep up with what the other devs are doing.
To stay in the channel when I'm not online, I use an IRC bouncer (znc running on a slice, for me). When I log back in on my computer, I have the conversation I "missed" played back for me. I know other people that use irssi in a screen session to accomplish something similar.