I've been using HipChat for a week now. Here are some short comparisons:
HipChat has a decent web interface. In fact, I think its interface is just sweet and just right. Not too cluttering, not too simple. It supports video and image embedding. It supports the hip memes such as the Rage FFFUUUUU face. These aren't supported in IRC protocol per se but are supported by clients such as Colloquy. Beside web interface, HipChat also provides desktop and mobile clients. If those are not enough, people can join HipChat with any Jabber/XMPP client such as Pidgin, and Adium too.
Functionality wise, HipChat is also pretty full featured. You can create group chat, and you can send private messages just as you could with IRC. However, you will be notified via email if someone mentions you while you are away. This is something IRC doesn't support and I don't know any IRC server that supports such feature either. At the same time, HipChat does not support things that IRC does. One of them is moderated chat rooms.
So, my impression is that HipChat is a nice offering for a company's chat. I would love to have it integrated with some pastebin solutions such that when you press Ctrl-C, if the buffer is long enough, it will be automatically redirected to a pastebin. That'll help so much with technical discussions. Furthermore, maybe integration with bug trackers would also be a nice feature to have.