TL;DR: Slack's interview process was one of the best I've experienced in over 10 years.

My total time in the interview loop was about 7 weeks. I was referred to them through a network contact, so my experience may be different, but it was:

• Phone call with a recruiter, focused on my experience and culture fit.
• Phone call with an engineering manager, a technical dive into my work history, challenging projects, etc.
• Coding exercise, reviewing code for a project that was completed by a mid-level-ish individual.
• Full "virtual interview," 3 and a half hours of meetings with various folks at Slack. Mostly focused on coding, team/culture fit, and systems design.
• Team match interview, meeting with three teams that had openings for my skill set. Each team was awesome, but one in particular was more aligned with my goals.

The recruiter I worked with was extremely helpful and transparent throughout the process. They gave me information on what to expect for each step and how to prepare for them as well. It was as if the recruiter actually wanted me to succeed! What a concept!

Review a pull request from someone.