That said, if I had the money and was invited to participate in this round I would have. More so than any other software I have seen, Slack has organic buy-in from everybody outside I.T. Once introduced is spreads like wildfire throughout any company I have heard about. Unlike much (most) software introduced to them Slack has an easy to see value for them, has almost no setup barriers, and is usable with little to no training.
All of the objections in this discussion are from an I.T. perspective ("you can just use self-hosted IRC for free") or a compliance perspective (having worked many years in the financial services industry I understand that there are compliance requirements but those are always written by lawyers and often their understanding of the security tradeoffs between self-hosted, with company employed security professionals, and the cloud, with vendor employed security professionals, are dubious at best).
I sometimes feel that programmers would write Excel from scratch in order to add two plus two.