1. Google has been doing this for years. It was known as "adwords". It's multi-billion dollars tool that gave birth to SEM.
2. It is limited to promotions tab. Promotions tab is where users go to check their promotional emails. I doubt Gmail will move them to real inbox. In fact, they will be foolish to do so. It's pretty smart move.
3. It is clearly marked as an ad with yellow background. So user is clearly informed about advertisement.
It's understandable that users are freaking out a little. But c'mon gmail has been doing ads in their inbox for a while. Email is extremely personal medium and they will be foolish to move ads from promotions tab to inbox tab. Google has lot of smart folks on their teams so we can trust them for the same.