Can anybody explain this valuation?
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.
But it's also the fact that while it's easy to get started, the value and dependence on Slack increases rapidly by your team's investment in using it.
Slack takes no time at all to do an initial team setup so you can just get started and as mentioned nothing needs to be learned to dive right in. Users are rewarded for discovering new things and further rewarded by cool points for sharing the tricks and secrets they uncover with others.
The ability to search and find a file or a key part of a conversation that happened a year ago faster than you could an email. This feature only gets better the more your conversations happen in Slack — you'll never lose anything again.
In terms of features, many of the best customizations involve apps working with Slackbot and Zapier. Setting up these custom features takes time and tweaking to get them exactly perfect — switching costs you won't want to deal with a year from now once they've become part of your routine.