Is there any web client you can easily deploy that comes close to Slack in comfort for the users? IRC is cool, but getting thousands of users comfortable with it sounds like quite a challenge.
That being said, their pricing is probably no better than Slack's for this purpose ($5/user/month).
Glad you like it, this year is going to be a big one for us - we're actively looking at how we can better serve large open communities, and I would suspect that will result in some nice changes to both free accounts and the pricing of paid accounts.
If anybody is interested in making IRC suck less, we've got a designer-shaped hole that needs filling... https://www.irccloud.com/jobs
I don't think it takes much to be comfortable with IRC, given the huge mix of people we have using it every day successfully. The IRC clients are really good at making users feel at home - Textual is set up for everyone in the company, although some people prefer to use Weechat.