Pros
Slack has a beautiful office in San Francisco. Almost everyone there is very nice and wants to see you succeed. The perks are nice, especially the weekly massages.
Cons
I had 3 managers (4 if you count the time I had no manager) over ~6 months. The whiplash was disorienting and I couldn't focus or get my work done. My original manager was amazing, his replacement was bad (overcommitted, late to everything, bad listener) to the point that I quit. I miss my teammates but I don't miss him. Additionally, when I was leaving, I found that multiple coworkers had tried to transfer out to other teams at Slack and had been denied by the director of our department. IMO, that's unacceptable. The pay isn't great - relatively low base pay for a company their size plus a yearly bonus that the finance department threatens to take away at every all hands.