[zyxley]

No, Slack is an IRC alternative (you can connect to it with an IRC client, but you lose many of the features) with assorted bells and whistles (emoji, adding reactions to posts, easy management of private rooms, etc), a bunch of integrations with other services, and persistent and fully searchable history.

That last one is really the key feature to me.

[nailer]

Sorry, I don't mean IRC client as in uses the same protocol, but rather that it primarily does what an IRC client would. I think it's great they've found success though.