I've never used it. What's wrong with it?
Other than that... It worked fine. I kinda liked it but slack is more robust for me (even if slow).
https://zulip.readthedocs.io/en/latest/subsystems/full-text-...
(It's only available when self-hosting at present.)
Turns out, it's a bit of both, and trying to use it as any single one of them, will probably make it harder to use.
Basically, I treat it as a live chat where it's easier to participate in different topics at the same time, Slack on steroids where everything is a "Thread" basically.
Alas, Flowdock is dead now and I've learned to embrace Slack as the better remaining alternative over Teams or Google Chat.