It's been useful already. One of the founders was out of town at the last YC dinner and but he participated in a conversation in the lobby with me and his cofounder through a Double. He wouldn't have been able to if he'd been using a standard teleconferencing setup, because it wasn't a pre-arranged meeting.

In my experience, many if not most of the most important conversations are not pre-arranged meetings. If you use existing teleconferencing technology you miss them all, and if you use a Double you don't. That's a qualitative change.