Slack is one of the first companies walking around with a billion dollar valuation that makes me go... "Yeah, maybe..." We use it. We love it. It's mature, well designed, gets improved constantly, and is used by more local businesses I know than not. It provides real, actual benefit to our small organization above and beyond what we pay for it.
Slack, to me, is a real product, with a real revenue stream and real value to teams. I really hope they never get bought and acquired or rolled in to a larger organization. The app is a constant joy to use - I can only say that about one other program I use daily.
For me, there is also a bunch of minor detail where Slack wins. Slack just looks better, behaves better with choppy connections, and has better code pasting support. It also does much better with automatic link extraction (HipChat's Linky is annoying and won't let you remove the automatic description). Lastly, Slack has integration playbooks that automatically uses APIs to configure stuff or (when that's not possible) gives you the recipe; for example, the Github and Zendesk integrations will automatically register the app and webhooks for you, almost no manual setup needed. HipChat is all manual.
And for the love of God, HipChat, fix your inane emojis. Immature 4Chan memes (trollface etc.) have no place in a business, not even among developers.
Oh, they do. E.g. to make fun of those trying to look sooo professional.