I've used it for talking to third-party clients that hire us and I don't see why should I revise my statement. It just works, it does the job just fine. The only complaint is that the desktop client melts my battery but given that I run it in the browser it isn't that big of a deal.

Also, I forgot to mention - we did have an Slack clone called Mattermost running and the overall response from the dev team has been positive. The only reason it was shut down is because of some open-source fanatics who thought Mattermost wasn't open-source enough (apparently the fact that it's backed by a big company providing support is a sin) so we've switched to this XMPP abomination.