really? $7 per active user per month? How little do you pay your staff? At the median wage, save an average of 90 seconds a day per person through better communication and it pays for itself. And it's pretty likely any team members using slack are at a 2-10x salary multiplier on that, a $100K engineer only needs to save 24 seconds a day...
As a company we are not short of options: Slack (the dev team liked it, but no one else used it and even a large part of the dev team didn’t use it), Google Hangouts (what everyone else used), Skype (no one here uses it), and now HipChat—which everyone in the company uses. This means that we weren’t choosing between $0 and $7, but:
• $0 (Slack) which provided something most people in the company didn’t use, and whose limitations we in the dev team hit fairly quickly (both 10k message chat history and the limited number of integrations).
• $0 (HipChat) which provides something most people in the company do use, and whose limitations we have only recently hit (the last six weeks) and are not currently bothering us (the limitation is just the 25k message chat history).
• $0 (Google Hangouts). Not really $0, but we already pay for it as part of GApps so it’s an incremental $0. The group chat functions suck, and there’s no integrations to speak of, but it is available.
• $2/mau (HipChat). Adds video/voice calls and screen sharing and unlimited chat history. We only need the chat history, and that isn’t bothering us.
• $7/mau (Slack). Just removes the 10 service integration limitation (which IIRC was 5 a few months ago) and the chat history and a few other features we don’t really care about but nothing to write home about. There’s also the $12/mau level and they are working on an Enterprise level.
There is absolutely nothing that Slack provides—except a bigger bill—that HipChat doesn’t provide as well for less. (And, even if Slack were better than HipChat, it isn’t 3x better. Sorry.)
This isn’t to say that Slack is a bad thing, just that it’s stupidly expensive and/or limited compared to other hosted options.