Since coronavirus Microsoft has the NHS (~1.7M employees) on teams.
I use both Teams and Slack and almost did a spit take at the idea of "switching" from Slack to Teams. I had noticed that Teams has more chat functionality than a meeting tool really needs, but I didn't imagine that it was intended to be usable as a company's primary chat system.
With that said, our company recently switched from slack to teams, and its been a fairly annoying change. Teams is miles behind slack in terms of usability.