Pros
People and culture has to be at the forefront. Slack has hired amazing people with abundance of different experiences and expertise. On top of that, we've hired some amazing people in general who are selfless and HYPER collaborative. It's amazing to see such a big team in CS all working in unity when it can be hard enough working with just one other person. Leadership has been one of the most impressive things to witness since joining Slack. Leaders at Slack do not just say thing to say things, in every instance, I've seen leadership lead by example. Very empathetic and compassionate leaders that sincerely care about your well being and go out of their way to assist you when you're down rather than tell you about it.
Kontras
Personally, I haven't been able to see anything that I can label as a con. If you're used to being in a startup and want things to move fast within a small team? Then perhaps this isn't necessarily that anymore. But at the same time if you're coming from a huge organization with 10s of thousands of employees? Things aren't structured to that point either. For me, it's a happy medium, but this could differ on what your expectations are.