Microsoft is smart to build their own Outlook killer, and they certainly succeeded in freaking Slack out[1]. We (SpiderOak) evaluated teams as competitive research when building Semaphor [2] (our own end-to-end encrypted alternative to Slack/HipChat.)
I found Teams to be a credible offering. It's shortcomings are mostly about being very new (e.g. no linux client yet, some unpolished UI associated with window resizing.) The O365 integration is nice. The client seems to be built using Electron and TypeScript, and if the pace of development maintains I expect it will be a serious threat to Slack. Of course Slack and Teams require trusting the vendor with your unencrypted content.