I was just about to switch to the new Mail in Lion and now this comes out. Thanks Google ;)
I'm a long time user of Google Apps / Gmail and this layout feels weird, uncomfortable and cluttered on my 15" MacBook Pro. It might be just that it's new and might take a few days to get used to the new layout.
Or may be it's not meant to be used on a 15" screen
The screenshot looks remarkably similar to the Google wave layout.
I thought Google axed Gmail Labs a while ago... Turns out there's more than one Labs in Google, and they're killing off some other one.
The new Gmail interface itself is ugly and then you add this to it...it's all gray everywhere and kind of hard to make distinctions. Don't even talk about using this new Gmail theme in lower screen size laptops...it's looks absolutely ugly...come on Google, I'm sure you can do better.
Looks like this will actually take advantage of the screen real-estate on a 24"+ monitor. Great!