As a hipchat using company, we got a slack account to give it a test drive, and I was ... surprised how much of a wash it is vs hipchat.
I had a "this is what all the fuss is about?" reaction.
I actually seem to prefer the hipchat UI personally, and apparently so do many of my colleagues. The fact that hipchat is cheaper only made it clearer to us.
note: only using and comparing the osx desktop app for both services.