It's not quite real time chat, but it almost is. You get the same attachments / decent UI benefits of Slack, it's open source, you can self host it without too much pain and it has a pretty nice API. No limitations on chat history too.
I think in some ways it's much better than Slack because you have proper categories and threads so I think Discourse is a million times better if you're looking to use it as a mailing list alternative.
In some ways it's worse than Slack because there's something very nice about just having to enter in 1 line of chat to get a message across, but with Discourse you would have to start a thread in some category.
I'm going to be using either Slack or Discourse to build a private community but I haven't decided on which one yet.
In case you've never heard of Discourse before, here's a live demo hosted on their official site https://try.discourse.org/.