Wow. Very interesting. At first glance, the interface looks very mobile friendly and also very simple.
I'm incredibly impressed. The only feedback I would give is that the buttons in the top bar could stand out a bit - maybe a different shade of blue or a slight border. It wasn't immediately clear to me where 'send' was.
The ads are a bit more obtrusive than Google Mail ads, though I think that would just take a bit of getting used to.
I'm impressed as fuck, though. I can't explain how impressed I am.