With the ArgTek antenna, using the three omnidirectional whips provided, and NOT the patches, I am seeing well over a country mile, or about 6,500 feet, with a solid video link, while flying from the crest of a gentle hill.
With stock antenna that came with the Phantom 3S controller, I was lucky to get half a mile of video and control connection, launching from the same exact spot. ArkTek is indeed a worthwhile investment that does dramatically increase my drone's range. I have no experience with any other range extenders.