I used it at a previous job and it was fantastic. The first-class threads are the way chat should work, the UX with the message river is brilliant, the keyboard shortcuts make everything two keystrokes away and it's extremely fast.

In comparison, I hate having to use Slack every day today. It's slow as shit, whenever I type a keystroke too fast (e.g. editing a previous message) it always gets into a weird state (usually edits the message before last and messes up the edits) and it encourages rapid-fire, thoughtless replies. Search also sucks and it's impossible to find anything. I fucking hate Slack.