Just had to download #slack for the first time. It's *bad*, but not *as* bad as I thought it would be.
For certain use cases Slack works really well. Particularly if you want company-wide chat, IRC just isn't very accessible for non-techies.
One area where Slack breaks down is really large communities with open enrollment.
Also, don't send anything super-sensitive over Slack in cleartext.
Have you played with Keybase's chat much?
I haven't used KeyBase chat (much, anyway) though I have it installed. With all these things, though, it comes down to what teams are already using—I'm not going to be able to show up somewhere new and get people to change their tools on day 1! And I'm not part of any teams that use KeyBase chat, at least at the moment.