4901 didn't choose to go with Slack, because we had a few too many users on dumbphones still. We chose GroupMe because it works over SMS as well, and that works well for our purposes.
That said, the
company I work for uses Slack all the time and it's wonderful. I was pushing for us to use it across the mentors of different FRC teams here in South Carolina; the real test will come next season, I suppose.