As a remote designer, this is a must have. I'm always contactable and available. Super easy way to also have quick calls and communicate with other team members on different teams.
I'm a part of a ton of Slack communities but I'm active in only a handful.
Slack works to its fullest only when it is implemented in a team and everyone's supposed to use it for communication. Slack could/should improve on making the communities on it better by adding relevant features.
I have used it extensively in Andrew Chen's talk platform: http://talk.andrewchen.co/ and also for Write The Docs forum at http://forum.writethedocs.org/. I think it meets most of the goals of a well structured, well monitored, and custom discussion forum. I like it.