I've switched over to using mattermost, it can be (and is) self hosted so I'm not as worried about slack getting hacked (again) and leaking our info/taking control of our systems with chatops.
The downside is; mattermost is not a strong replacement. I wrote a bot to basically do a large part of my job for me, and coding against mattermosts websocket protocol and dealing with authentication has been messy to say the least.
Slack is a joy to code against and use from a UI perspective when compared. (sadly, as I don't like it for ideological reasons)
Here's a fun one: say I use the MIT binaries, then debug a problem by reading the AGPL source code. What legal position am I in? (If you have an answer for that rhetorical question, by the way, you haven't thought of the problem long enough.)
https://github.com/mattermost/mattermost-server/blob/master/...
They launched exclusively AGPL and then made MIT as a concession after discovering that a number of companies outright ban AGPL and won't pay for it unlike MongoDB, but licensing binaries differently than source code is not something you come across often.
On a side note, Last Chance to See is a great book (Douglas Adams) and show (Stephen Fry), highly recommend both of them.