I have heard good things about Slack, but I really don't see how it doesn't end up just being "another thing that I have to check". We just use email lists for group communications, because are already checking their email for school.
I suppose if Slack can notify you of updates via a phone notification, than it could be a little more convenient than email, but then I think that being flooded with notifications could be a problem. And even then, you only get the benefit of phone notifications IF each student has a Smart Phone. If students without Smart Phones have to log in to some Slack web view, then it is more of a hassle than simple emails.
Do teams who use Slack pass information along to parents at all? If so, is that through Slack too (I would be very hesitant to try to force some parents to use it), or does that go through email anyway. Then you are managing both emails and Slack channels.
I would love to be proven wrong
I'm just not convinced of the benefits yet.