The UI looks a bit outdated, but it's so fast and usable that I don't mind the aesthetics at all. It had some annoyances like the inability to move topics, bit AFAIK they solved that with the latest release.
There isn't really much else I don't like about it, I never stop being amazed by how great the keyboard navigation is and how much the "river" of messages improves usability.
Yeah, I've used that and it worked well (though I haven't used it extensively). As long as clients don't get confused by the UI, it's great, but it might take a bit of onboarding due to the different UI (though in its core it's just "Slack, but everything is a message thread").
There isn't really much else I don't like about it, I never stop being amazed by how great the keyboard navigation is and how much the "river" of messages improves usability.