This feature is such a game-changer - I take the occasion to plug my own Slack app Smooz[https://www.smooz.io] which does the same thing, creating shared channels, even if it will probably be replaced by Slack native feature when they extend it to free plans
When it came out, it was really a big deal for our users, so I can understand why Slack worked so hard to add this feature. The engineering description is fascinating. Also, while it's never fun to see your app being taken over by the platform, I must stress that Slack API team was very fair and gave me a heads-up far, far ahead of release