We use it daily (around 200 people) and have replaced several slack licenses and Skype for Business. We figured it was a good way to unify the organisation under one platform that was already paid for with our 365 license. Not everyone is happy losing Slack, but overall, I've found it to be a massive improvement over Skype for Business, particularly its user friendliness across different platforms (Windows, Mac, iOS, Linux). Its been pretty solid in terms of conference calls, group creation and project organisation (conversations, file-sharing, etc)