I thought I'd take some time to give a real review instead of just complain about the cost.
Duet works as advertised and since it relies on a cable for the connection, it works quite well. The screen updates very quickly and most things look and animate well.
You install the free Duet app on your Mac (no Windows support yet, though they say that is coming) and buy the iOS app. The Duet app on the Mac is a regular application, but when you launch it, it will just ensure the Duet driver is loaded. If it isn't, you will be prompted to install it and restart your Mac. Once done, you launch the Duet app on your iOS device (it works with any iOS device that has iOS 7 or later on it right now). Then you connect your iOS device to your Mac while the app is running. A moment later, the Duet driver on your Mac should see the iOS device and set it up instantly for use.
On your Mac there is a Duet menu bar widget where you can see if an iOS device is detected, and if so what it is. Then you can click the small gear in the lower right (it is light in color and may be easy to miss for some people). The gear lets you control the virtual size of the display (regular, non-retina by default; retina (for retina-capable iOS devices - increases the pixel count without changing the apparent size of the virtual display so everything looks crisper); and high resolution which increases the pixel count but keeps the interface 1-to-1 so everything is much smaller but you can fit more on the virtual display). You can also control the frame rate (30 or 60 frames per second), the general battery efficiency of Duet (which seems to auto-adjust settings as needed) and a few preferences (such as whether Duet launches on startup or not.
Next, go to Displays in the System Preferences on your Mac. There you can set the location of your iOS display relative to your other display(s).
Finally, just drag an app over to the iOS display just as you normally would.
Duet currently doesn't support more than one iOS device connected at the same time (though you can swap out devices).
Performance is good, but don't expect to be playing 3D games on the iOS device. It doesn't appear to support Quartz Composer, so most screen savers won't work for example. However, the screen will blank normally.
Most videos play just fine. You may notice a slight flicker at first, but it usually plays without a problem (at least with the high performance settings, anyway).
If you use Yosemite 10.10 or later on your Mac, and iOS 8 or later on your iOS device, you can actually use the QuickTime Player on your Mac to take a screencast of your iOS virtual device. However, to get this to work, I first had to go to the home screen on my iOS device, select my iOS device as the camera source and then re-enter the Duet app. Also, taking screenshots on your iOS device works just like normal, so you can take shots of your virtual display screen. These can be handy features for those of us who may need to train others or show them a screencast of how a feature works.
Sound does not appear to be passed to the iOS device, so if you were hoping to use it to listen to stuff on your Mac, you will be disappointed. Duet only does video.
As for interaction with the virtual display on your iOS device, you tap on the screen to instantly make the mouse pointer appear in that location. Double tap to "click" on the item. It works OK, but since your finger is larger than the mouse pointer, it will take some time to get used to tapping in the right spot. Because of this, I usually just tap once to move the mouse to the display and then use my regular mouse to click around and do stuff.
The Duet app on iOS does seem to run in the background, so the virtual display doesn't disappear from your Mac if you need to check another app on your iOS device, but if you close it (or if it closes automatically) then the display will disappear from your Mac and the items on the iOS device will be moved onto one of your main displays, usually the one closest to the area where your iOS device was virtually. However, if you re-open the app, the items that were on it will reappear there. That being said, I have noticed that if you close the app on your Mac that was on the iOS virtual display and then relaunch the app on your Mac, the windows displayed won't appear on the iOS virtual display. They will appear on one of your other monitors and you will have to drag the window back to the iOS device virtual display.
The iOS virtual display supports the new "each screen can be a separate desktop display with a separate dock and menu bar" feature of Yosemite, so you can put an app into full screen mode and it will fill up just your iOS device virtual display.
At the moment, making some apps full screen on the iOS device doesn't seem to behave like you'd expect, though. Full screen apps will extend a bit off of the side of the iOS device so you cannot see all of the app. It appears to extend about 1/5 of the app off the screen. To avoid this issue for now, just Option click on the small green dot at the top of the window on the iOS virtual display. This will extend the app to the edges of the viewable border, but not beyond that.
If you change the orientation of your iOS device, Duet will blur the screen temporarily and adjust the size of the virtual display accordingly. Everything currently on the iOS virtual display will remain there, but all of your displays connected to your Mac will blink when the virtual display size and resolution are adjusted.
The Duet iOS app appears to automatically disable the auto-screen dimming and iOS lock features while the app is in the front. This is generally what you'd want, since having to unlock your iOS device constantly would be a pain. However, you should keep that fact in mind if you get up and walk away from your desk (for security reasons).
All-in-all, Duet does a much better job than I expected from software like this. It generally just works the way you'd expect. Keep in mind, though, that this app does NOT work via Wifi, so if you don't want to tether your iOS device via a cable or dock, then Duet is not for you. Look at AirDisplay. Of course Wifi performance will never be as good as a direct cable connection, so even if the developers of Duet were to add such a feature, it likely would not perform nearly as well as the tethered version does.
Mac app: Free
iOS app: $14.99 as of the time of this view
* App seems to balance performance and resource use well.
* You can take screenshots or (on Yosemite and iOS 8 or later) record video of your virtual display using the QuickTime Record New Movie Feature.
* Full retina support if your iOS device offers it.
* It generally just works the way you'd expect it to.
* Since the device is tethered, performance is snappy and you don't have the sort of security concerns you might have if Duet worked over Wifi or Bluetooth.
* Continues to run in the background if you need to use your iOS device to check another app temporarily.
* It only supports tethered use, there is no WiFi or Bluetooth support.
* It doesn't support more than 1 iOS device at the same time (though they plan to support multiple devices in the future).
* Full screen apps currently seem to extend a bit off of the iOS virtual display.
* No sound support.
* No Windows support yet (though they say that is coming).
* No support for iOS devices that cannot run iOS 7.