Initially the prices favored Amazon, but as of late, I've been finding things (search: "training pants 3t") where Amazon is 50%+ more expensive than the local stores through GSX. The same day experience is mind-blowing; I don't stop off at the local hardware store, I search for my garage door opener battery model and it's on my back porch by the time I get home.
Everyone's had a guffaw about Amazon's automated pricing agent and the $23M book [1], but Google is in a position to really trip them up, unless Amazon gets a reign in on it's pricing. And then there's the fact that "2 day shipping" pales in comparison to Google's same-day experience.
I was lucky enough to be in an an area that Google serviced, and would probably have also tried Amazon Fresh, but honestly, the free beta was too good to pass up (and I would have thought twice about the $299/y Fresh subscription though I'm sure it's worth it).
My only concern is that the deliveries are made from stores nowhere near me really and I'd be much less guilty if the same brand stores were more local (they're losing my business as well right now).
[1] http://www.pcmag.com/article2/0,2817,2384102,00.asp
(edit clarity)