My opinion(1), after playing a while with MS Teams:<p>The pros : the product is quite good, let say it is good enough and much better than the status quo of relying on emails or Whatsapp. It is integrated with the Office suite, and if I understand correctly, will be free for pro Office users. It has a handful of very interesting product decisions, like threaded replies and iframe apps.<p>The cons : MS Teams is not as polished as Slack, the UI (especially of the chat area) is not as clear. The installation/onboarding is not as fluid. They have less apps and integrations (obviously, this is just the beginning).<p>My guess : MS Teams will not even begin to make a dent in the almost 100% market share of Slack among startups, agencies and similar small, agile groups. The whole Office ecosystem is simply to heavy, the UX is not as great, and some integrations might be missing. On the other hand, it might win big in the entreprise segment, where Slack is less present, and where being in the Office family is not seen as a drawback but actually an sign of solidity<p>(1) I am an independent dev, working on an app for their platform, so this is an educated guess based on my testing. I will enable Teams to connect with each other or with Slack teams through shared chat (www.smooz.io/microsoft).