This is a horrible way to think about it, slack doesn't need to deliver 4x the value, it just needs to deliver $6 worth of extra value per month.

Even at the very cheapest end of the spectrum, a developer is going to cost at least $60 an hour, fully loaded. If slack can make them 6 minutes more productive a month (ie: 12 seconds a day), then slack is worth it. These are productivity differences that aren't even measurable in practice which means the rational thing to do is to just be price insensitive and adopt the tool that works best, regardless of cost.

Yes. The integrations. https://slack.com/integrations

This isn't the usual "host your own server" integration. All Slack asks for are credentials for services you use and it seamlessly integrates into an assigned chat room (channel). My CEO did it for the startup I work at and in ten minutes from signup we had: Github, Trello, New Relic, Twitter, and Stripe all integrated.

Hipchat does that too

edit: To clarify, we run github freshdesk and jenkins integrations. I'm sure there's plenty more. From what I recall it works in a similar way too.

Can't speak to all of them, but the Github integration with Slack is better than HipChat. The messages are more useful and easier to customize.

Slack has IRC gateway, so you can use irssi for company chat.

It might not be a big deal for you, but when we were briefly using Slack at previous company, I loved that I could just add it as another network to irssi.

Unfortunately, the IRC gateway causes a bad experience, not so much for the people using IRC, but for the people using the real Slack client. Slack apparently doesn't recognize username mentions ("@joe123") coming from IRC, so everyone needs to add their username to their list of highlight words so that the team members using IRC can mention them.

One could use Bitlbee to connect irssi (or any other irc client) to a jabber server and not only.

http://www.bitlbee.org/main.php/news.r.html

It really depends on what you want to get out of chat $2 is a good price to pay if your team chatting needs are minimal. But as your organization grows, especially if it starts to involve a larger team and projects with larger scope and interdependent pieces, your chat needs evolve into more of a communications need. In such cases it may be worth it to look into a more comprehensive chat tool since it gives you a much more powerful communications platform than your AIM or even email.

If a significant chunk of your company's discussion is taking place in chat rooms, it may be time to look into an upgrade.

You'd want something that gives you some standard next-gen functions: great global chat history search,video conferencing, screen sharing, markdown support, distributed team presence tracker, advanced noise management tools, dedicated integration efforts (integration to basic collaboration tools like asana, trello, etc should take minimal effort)

Keep in mind Slack is hardly the only other alternative in this game. Zapier gave a really good overview the other day, including Kato, who (full disclosure) I work for. https://zapier.com/blog/best-team-chat-app/

also @shalmanese makes a good point about justifying the cost.