Sure they can bascially SPAM their current users, but then again I think that people hate it when the internet does that. For example, I go to a website, and they will have advertisements directed towards me for companies in Michigan. I don't like that.
From a business point of view, its a great idea, but from a personal point of view, I hate it
I think it will be interesting to see how it works out. If they can keep it from going over the top with spam, then maybe it has a chance. The people are Google have seemed to be on the bright side. If it does work and pays for the free wifi then that will be incredible! What a revolution it could potentially be.
The down side is that you will be tracked where ever you go.