We'd have picked slack over teams in a minute, though it has compliance issues making it unusable for us - slack is just more polished and has a lot more that you can get out of the box / with plugins. That said, teams still has some great features for us, so we'll stick with it for at least 2017. We've tried a few similar options before (primarily group chat) but they all had gaps / issues that resulted in them being not as useful/used as we'd have liked.
There are still a couple of massive gaps in teams for us:
* No on-premise TFS integration
* It seems that Office 365 is all or nothing - we can't tie the o365 account to our primary email address without it messing up external email routing.
If you're trying to replace a culture of email + some IM + files tucked away in some document management system that don't get updated as frequently as they should, it's a good start. If you've already got good communication then it's less valuable.