It's quite a bit different from group chat in Gtalk. The biggest difference is that the rooms are persistent and any time you log in you can scroll back to catch up on missed conversations. With Gtalk (or any IM service) group chats are usually ad-hoc and treated more like meetings: you start one, talk for a while, then close it. The idea here is to have a constant stream and have topic- or team-oriented rooms where people can always go to ask questions or get ideas from their coworkers.

It is, however, very similar to Campfire. Right now, the advantages over Campfire include performance (which is actually quite key for real-time conversations like this), and the ability to sit in multiple rooms at once and receive notifications about messages to you in background rooms (also pretty important, we've found). That said, we're in beta! So, we definitely plan on differentiating ourselves more in the coming months.