We've been using Zulip for just over a week in xi-editor and related projects, and I'm thrilled. Huge improvement over the pastiche of IRC and Reddit we were using before.
A bunch of other cool open source projects use it, including Python Core, parts of the Rust community, MariaDB, Coala, the Lean Theorem Prover, the HL7 standard, and many more.
That's what I did. Thanks again to them, I really appreciate it.
Reddit was OK for awhile... I dislike the redesign because there are bugs with code formatting, and because the latency is worse. And it seems like they couldn't really make up their mind visually.
Actually Zulip is one of the lowest latency web apps I've used in recent memory!
I also have problems following IRC because of the lack of threading.