I was interested in this question since I was considering buying an ADS
-B receiver like Airnav's radarbox so as to know what the flyovers were. I subscribe to the planeplotter network and see about 5 - 15% of the overflights as having ADS
-B. I decided against purchasing it for now. I believe in a few years most commercial aircraft will participate and will re-visit the notion of purchasing a radarbox type receiver at that time.
An interesting selection effect is that since foreign carriers are more likely to use ADS
-B, they tend to disproportionately show up in detections! Also, such flights are more likely to be detected near major destinations and along routes linking them such as NY - LAX