It takes about a week to get used to, then people don't want to go back.
Bonus: it can be hosted for free (incl. SSO), though I take the path of paying the authors for their hosted version.
I really don't think I'll go back to Slack; Especially as they're owned by Salesforce and I suspect even more lockin in future and especially rent-seeking behaviour- which always comes from these large acquisitions. (people should be mindful of lock-in for those reasons in my opinion).
I also get massively tilted when non-optional changes to clients happen, like when they completely broke markdown formatting for their style bar, yet still had some markdown formatting enabled which messed up your messages randomly; like not being able to exit code blocks. >_<