On to the service itself:
I can't join in on my client's conversation without creating a whole new account with a different email address. Annoying and incredibly short-sighted. I know that the people over at HipChat never work with anyone outside of their office, but unfortunately this is not the case for most people.
Honestly, I can get an IRC server up and running in the time it takes to get a HipChat account activated. It's annoying that so many people are hyping HipChat, as if it were some groundbreaking thing.