I like that it's open-ended but structured enough so we can assign tasks when needed without having too much clutter in our inboxes.
The interface could be made more user friendly if they're still working out some kinks (like how often messages are sent via email instead). Also, there should definitely be an option to attach files directly from within chats! We've been using this as part of a work group at my organization - both internally between employees who use Slack regularly, and externally among members outside the org.