I just spent $10 on Duet Display to turn my aging iPad into a secondary MacBook display. That’s about five times more than I’d normally consider prudent for an app, but it was worth every penny.
I’ve been a daily iPad-as-secondary-display user for years. My go-to app had always been Air Display (and then Air Display 2). That just changed. For the same price as Air Display 2 I’m able to ditch the unstable Wi-Fi connection typical to these remote display apps and extend my workspace over an old 30-pin USB cable. That's given new life to my three-year-old MacBook Air and three-year-old iPad 2. No stutters, no lag, and it's surprisingly robust enough to support video playback. Duet says its app was developed by a team of ex-Apple engineers, and it definitely seems to benefit from this expertise.
After getting past the price, setting up Duet was painless. I was up and running in five minutes after first downloading the app from the iOS App Store and then installing the free companion app on my MacBook Air (and rebooting). Duet found the iPad just as soon as I connected the USB cable. The company claims it can deliver a retina display at 60 frames per second with no lag. That might be true, but I’m running in energy saving mode at 30 fps, and I’m still able to watch Kim Jong-un’s head explode with glorious fluidity while working from this cafe. Sure, my set-up is nerdy as hell but any embarrassment is offset by my increased productivity, so the choice is a real no-brainer.