The work environment is great: offices are incredibly thoughtfully designed spaces, coworkers are smart and caring, benefits are well above average, and the work is meaningful (ok, that last one is subjective, but almost everyone here believes in collaboration, openness, and a future where nearly every company uses a tool like Slack).
Cons
The primary (and only major) con is that the company is still trying to figure out how to operate at its current scale: there is friction between teams as goals and priorities are not always clear, the work planning process changes every quarter, and there is some expected thrash as projects are not always smoothly initiated or completed.
I've been at Slack for 9 months and I'm really enjoying my job. They hire top quality people and they try to live the values - smart, humble, hardworking and collaborative. People are appreciated for the work they do and it's possible to have a large impact on the company.
Cons
Lots of change - I've had 3 bosses in 9 months. Hasn't had much of an impact on my day to day life, but it's a bit thrash-y. The way we use Slack can be overwhelming - I must be in 100 channels. I look forward to the day when are priorities are to make the product easier to use for Slack-centric companies. Sometimes the best way to get more work done is to turn off Slack.