We examined both Kato and Slack for internal use at our company a while ago. I personally preferred Kato's split pane chat windows and handling of multiple team (which Slack still doesn't do right). We also thought it was more competitively priced. After we had made a case internally for team chat, we started a full department trial of Slack, at which point point our users became fairly enamoured with it, and probably unwilling to uproot and change to a new service.
I have a feeling this matches the world at large for Slack. By implementing a very personable interface users seem to grow a positive emotional connection to the service. This reflects in their usage which is also positive and casual, encouraged by features like custom emoji. All this comes together into not the most practical and functional features on paper, but definitely the most enrapturing user experience.
A good read if you want to learn more about Slack's on going world domination: https://medium.com/@awilkinson/slack-s-2-8-billion-dollar-se...
