The 7 years that google owned zagat correlated with a massive improvement in the quality of their reviews.
I have done the google maps local guide program a bit and the way it is structured is great. The google map slowly switches into proactive mode and solicits photos, ratings, and reviews.
A few months ago a friend asked me what I was up to, so I sent them a photo of 2 flights of beer in an attempt to get them to meet me at the bar. later, Maps asked if I liked the bar and wanted to upload the photo. For the last 2 months that photo has been the official picture of the bar and received over 250k views. Minimal input or interruption for me, large return for google.