Yes, we switched over to Slack from IRC/Google+ at elementary (elementaryos.org) in the early summer. It's been a massive improvement. We're starting to feel the limitations of the free tier though, so we do wish it were a lot cheaper.

Could you enumerate the particular pains you had with IRC that were eased by switching to Slack?

I can't speak for him, however as a programmer speaking to other programmers, I find Slack is pretty nice primarily for these reasons:

Pasting code snippets, preserving formatting and getting syntax highlighting. Pasting screenshots, usually of code, error logs, or graphical bugs. Inlining code with `def func_name()` or whatever, making things much more legible. Easily sending files.

Basically all code manipulation at their root.

For actual normal communication, IRC works just as well for me.

If you host your source on GitHub, you should definitely check out Gitter. It's free for public repos and the integration is great; you get Markdown, integrations, IRC bridge, etc. Check for example the Marionette community to get a feeling of it: https://gitter.im/marionettejs/backbone.marionette. Disclaimer: I work at Gitter, feel free to ping me at https://gitter.im/malditogeek

Check out https://echoplex.us