Used it for about 2 years at a growing startup. What I liked:
* Threads are way better than in Slack. They're top level instead of just a bolted on afterthought. This means all your various conversations scale way better and are way easier to find than in Slack. I can't overstate this enough, it's a killer feature and genuinely improves the overall organisation of your communications
* Font size is just slightly smaller. My eyesight isn't what it used to be, but I still think they get the balance of legibility and information density spot on, whereas Slack feels cartoony in comparison.
* Search felt a little better, I can't exactly put my finger on why or how, just that finding things in Slack always feels comparable to the iOS Mail search feature: very basic.
Drawbacks:
* There are less out of the box integrations I think.
