The reason why we started using Slack is simple. Its simpler than IRC and accessible to non techies and it's free (compared to HipChat and others from that time).

Not marketing, or features or UX - price was key for us.

I still hope Slack wins, but MS is a very serious threat. In the end its best for us, users - prices will drop, new features will be added, support will be awesome all thanks to competition.