The major benefit of slack over hipchat is slack comes with integrations by default. IE: instead of having to use Zapier or such to hook into Trello slack can handle it for you.
HipChat only has an API you push to, whereas Slack can pull from other services.
We tried it and kept with HipChat. I found Slack's interface/text to be less readable than HipChat. It might look better, but for reading information and usability HipChat won. Our entire team is happy with our choice.
HipChat integrations can be frustrating/lacking and I often find myself resorting to Zapier. The integration with JIRA is a joke and a huge pain to setup/manage for large workflows, which is especially surprising since HipChat and JIRA are both part of Atlassian.
I do like the newest version of the HipChat Mac app a lot though.
I'm not sure the integration frustrations merit the 4x cost increase in switching to Slack though. Hard to justify that unless your team is very small.
