We switched for largely arbitrary reasons. I do really like the Slack experience, I'd say I prefer it in some ways. But Flowdock was also great, no real complaints there at all.

A sign that the switching costs are maybe not that big. We went from campfire to hipchat to slack. And each time we were happy to move on despite a history of chat messages (old documents aren't that useful since anything important is in Dropbox). Interesting phenomenon.