At our company we had used hipchat for a little over 2 years, back when it was charging for when you had more than 5 users. I had previously tried to get the founders to try slack (since it was all the buzz) and failed. Since nothing was broken, I decided that hipchat was good enough for the team.
Recently, around the beginning of this year, one of my co-founders had become very impressed with slack as a company and as a product. He gave it a whirl and got the team to decide to switch to slack. Personally I wanted to try out slack and with the recent outage they had and friends jumping ship, I was pumped to make the switch.