Off-Topic Curiosity: A few time ago, a new bug in bugzilla appeared to request that Mozilla Panorama's tab groups were window based (I wish I had that bug's ID). This would make their over-engineered tab group system much simpler and intuitive, as each group would simply be a window, literally. Unfortunately, the design change is deep, and so it won't make it for Firefox 4, if it is accepted at all.

About the topic itself... I like the idea, but I certainly would need to get used to it (just like I'd need to get used to Mozilla's Panorama). The only thing I'd like to find out is if there is some way to browse trough collapsed stacks without having to expand them, just like (using Opera's analogy on the video) people grab piles of paper, check them quickly and take the one they want, without having to display all papers over the table just to choose one.