The Air app for desktop can sometimes have performance issues for no particular reason. They're about to release a native OS X app though so that should solve this for a large amount of users.
The android application in woeful. It consistently logs me out and forgets my password which means that it's totally unusable for out of office notifications from people. I've had to rely more on their email notifications for unread personal messages than their Android app. It's a real shame because I could see myself getting a lot more use out of Hipchat if it worked well enough.
The best way to access hipchat that I've seen so far was just a jabber client - xmpp seems to be the backend of their service.
And my coworkers really love the web client. And some use xmpp. Many of us regularly use the iOS app. I guess that's one of the strength's of hipchat, the variety of access methods that all feel basically first-class. (and one person on our team even uses the SMS integration.)