To me, this looks like a reskinned Gmail. If it were that, then yes I would use it as it's an improvement over the current layout, albeit marginal. However, there are no ads which Gmail needs and imagine if Gmail didn't need adds they would use their real estate more wisely as in your design.
I use Chrome, which allows you to add styles to custom.css to override the look of any website. As such I'm able to add my own styles to customize the look of Gmail to look cleaner, hide ads and customize like in your mockup. If you package the new look as a Browser Plugin to restyle Gmail, then yes I would use it, but now I would not pay for it.
If you are attempting to recreate gmail itself, then also no I would never use it. Because once again this only offers an improvement in design. Gmail is great compared because of Google. Aka, best spam prevention, far better than Apple, Yahoo and Outlook and scaleability and up-time. There is simply no way you can compete with the resources Google has. Which is why even Apple who has incredible resources and Money and amazing design sense still can't compete with Gmail's infrastructure and servers.
There is a market for retheming however, whether in the web interface or through Apps such as with Sparrow.
So in reality stick with theming Gmail and be aware that Gmail can change their look at any time and that you won't get any direct money out of it but will get your name out there. Good luck.