Opt-in isn't quite enough for me, but it is a step in the right direction. I also need to know that, should I enable these location-based ads, these ads will not spy on me. I'd also like ads that don't suck up huge quantities of processing power, and don't interfere with my ability to use my own device. These ads must be reasonably discrete, which I realize is something the advertising industry hates doing.
Also, just because something is opt-in right now, doesn't mean it will remain so in the future. I'll bet Google will make it an opt-out later. Then, they'll make the "opt-out" less of an actual opt-out, and more like a fake out-out, much like what they have done with have done with their web-based advertising.