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.