Ask HN: How does your company use Slack?

I'm writing a report for a class on how companies use Slack, how it affects their productivity, etc. It would be incredibly helpful in anyone who works for a company that uses Slack could provide answers to the following questions:

1. How has Slack affected your team's productivity?

2. What are your top 3 features on Slack?

3. Why did you choose Slack instead of some other form of interoffice communication system (Office communicator, Skype, etc.)?

4. Are there any disadvantages to using Slack?

Thanks for the help!