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From my experience with Rocket Chat I'd argue it's about a 30% implementation of Slack when it comes to features and polish.

My favorite gripe with it is that quickly typing half a username and auto completing will select the first (or some random) user that's doesn't have anything to do with what you typed. It's a real pain if you're a fast typist.

Mattermost is the only one I've used and it's both easy to administer and works great.

I use mattermost at work. It's terrible. Coworkers don't seem particularly impressed with it either.

Can't compare it to Slack though, since I never used it.