Isnt this pretty much the same as what firefox "tree style tab" addon has been doing for some time already? Except for the preview option, but im not a big fan of that anyway. And tree style tab was putting the tabs on the side, which imho is better because it saves the precious(on widescreens) vertical space.
Opera has been able to move the tabs to the left side for a while. So combine that with this new feature and you have the basic functionality of that "tree style tab" addon except only 1 level deep but much more polished/usable.
More usable? I'd say definitely not. Tab-style-tree works automatically in the way I like it to the best. If I want to further group things beyond what it automatically does, I grab the tab and move it to where I want it - I don't have to hover there until it snaps to stacked mode.
Furthermore: with Informational Tab, you get thumbnails on all your tabs; with Multiple Tab Handler, you easily select many tabs to move to where you want them. These three plugins work like a perfect trio.
Even better, if you don't like the default behavior, it's highly configurable based on how the tab is opened, and from where (although I don't believe site specific options are available).
It does look polished, and having not tried the new Opera i would not argue about usability.
As a general consideration, having nice window previews sure looks cool(except if they are too small, in which case they look really messy), but i dont think this is an usability improvement. Mostly because a consice title would tell you just as much - in a tenth of the space. Coolness gets confused with usability a bit too often imho.
Anyway, good to see the cool concepts spread :)