People are making the valid comparison of Hipchat and Slack, which is to be expected. But, this is a deeper move by Atlassian. Atlassian's business model is still "All roads lead to JIRA/Confluence." It's why so many of their products are free. This isn't as much a Hipchat v. Slack/IRC/Google/Campfire move, as adding another road to JIRA.
I just switched our team to JIRA for this exact reason. We were already on Bitbucket for repos and using Sourcetree for git access, so might as well get issue tracking from the same place. Honestly my feelings on it are mixed so far. Doing a "good" job of administering projects and issues seems to have a pretty steep learning curve if you want to use more than about 10% of JIRA's functionality.
That's why I like Youtrack more. It's far less complex and driven by search plus keyboard shortcuts.