They haven't actually shown off their vertical tabs yet, so there isn't much to say.
I am a Sideberry user though, and this excites me because a native implementation will likely look a lot cleaner, integrate better with your chosen theme, and actually allow the removal of the regular tab bar (you have to keep it with Sideberry because there is some functionality that cannot be replicated through an extension).
Theming in particular might be a big one, I had to spend a couple hours tweaking the Sideberry CSS to make it look nice with the skin I use in Firefox. Not having to do that sure would be convenient, especially for the average user who doesn't know what a stylesheet is.
