I'm in an academic lab that uses it, I think because it's included free with the university's 365 subscription. I prefer Slack, I find the channel organization and notification system more sensible and Teams to be a bit buggy. I also find that it's easier to miss messages in Teams with the way it collapses threads by default, but maybe I just haven't adapted yet from Slack's IRC/chatroom model.

Not an "important" issue, but I also miss the built-in emoji auto-complete.