Pros
The recruiting process was the best I have experienced so far: transparent, proactive, organized, with timely feedback and they give you the opportunity and even encourage you to speak to individuals about their experience working at Slack before you decide whether or not to join.
I´m sure Slack differentiates by this very well executed process alone.
In addition, I really liked the different steps of the process and the format of the conversations they conduct to both get to know the candidates but to also to let them take a glimpse of the company culture and values.
Cons
I´ve not come across a single negative thing yet.