Hardware Pick: Netgear Orbi
Leo Laporte has been trying every mesh Wi-Fi product under the sun, including the Google, Eero, Plume, and more. The latest he's tried is the Orbi from Netgear, which is his pick of the week from This Week in Google 384. This is not quite a mesh network. Instead, it's a more traditional AC3000 Netgear router with lots of power, and a satellite that uses a separate backchannel to avoid the problem that repeaters have of sharing the bandwidth for intersatellite communication. Since it goes around the band that you use, it's able to have high speed communication with the satellite.
If you've had any difficulty with the newer mesh Wi-Fi network products, this would be a great alternative. However, unlike those newer Wi-Fi systems, the Netgear Orbi is managed more like a traditional router. There's no app for easy setup, and all settings are configured in a web interface using a browser like a normal router.
Netgear sells 2 Orbis to cover a 4,000 square foot house, and they do sell additional ones individually.