I still really like HipChat but the latest versions have been unbelievably bad. Connection issues, message delivery issues, users showing up as offline who are actually online, etc... If they don't fix things soon we're going to have switch to Slack.
I kept getting logged out a couple of times a day, HipChat support recommended reverting to 3.X and "ignoring the upgrade popups". That didn't really help. It times out over night so the first thing you have to do in the morning is remember to "refresh" your HipChat connection. It doesn't flag conversations as having new content while you are away so catching up means visiting every room. It flushed my configuration a few times, so I had to reopen all the rooms I'm in and reconfigure all my settings.
Basically it became I tool I maintained instead of a tool that made my life easier.
Now that we are no longer using HipChat, the buggiest application I use on a regular basis is Skype.