What are some alternatives?
When comparing Air Display and spacedesk, you can also consider the following products
Duet Display - With Duet Display, you can transform your iPhone or iPad into a second display to expand the screen space of your PC or Mac system.
TwomonUSB - You can create dual monitors quickly and easily using a USB.
iDisplay - iDisplay is an app that turns a mobile device into a secondary desktop monitor. The app currently supports iOS, Android, Windows and MacOS, and it lets users connect a desktop computer to a mobile device over WiFi. Read more about iDisplay.
Splashtop Wired XDisplay - Use Splashtop Wired XDisplay to turn your iPad, Android tablet, Kindle or smartphone into a high-performance extra monitor for your computer.
Multiplicity - Multiplicity enables a user to control multiple computers with one keyboard and mouse.
Mountie - Mount a second screen (iPad/iPhone/tablet) to your MacBook