Turns iPad/iPhone into a second monitor
Works with Mac and PC
Simple to install and configure
Somewhat slow response
Moving onscreen items causes visual artifacts
No Retina Display support
Recent productivity studies have shown that adding a second monitor to your computer can increase your productivity anywhere from 9-50%. While increased productivity may be appealing, not everyone wants to spend the money for a second monitor. But if you've already got an iPad or iPhone, and a spare $10, you can have a second monitor using Air Display.
Air Display combines a desktop program for Mac and Windows with an iPad/iPhone app. As long as both devices are on the same WiFi network, your iOS device can become a second (or even third) monitor for your computer.
Getting Air Display up and running is fairly easy--install the desktop and iOS apps, make a few simple configurations of the program (in most cases; more complex computing setups may require more configuration), let your computer add the additional screen--and you've got a wireless monitor.
Useful in Limited Cases
Once Air Display is connected to your computer, you can drag any window from your main computer screen to your iPad or iPhone. Air Display pulls off the neat trick of allowing you to interact with those windows by mouse or--using the iPad interface--touch. This means you can drag a calculator onto the iPad and enter numbers right onscreen, instead of using the mouse or keyboard.
Not only is being able to interact with your computer by touch pretty neat, the ability to move simple programs onto your iPad and then walk away from your computer (keep a chat running while you leave your desk and make dinner, for instance)is impressive.
For programs that require more movement and have more complex interfaces--a web browser, for instance--the experience is a bit more mixed. Air Display's performance lags in this area. For instance, when you move the window of a more-complex desktop program, the window gets blurry and visual artifacts appear, though Air Display quickly recovers and regains its good looks. There can also be a slight lag when moving these windows.
Given this drawback, Air Display is probably most useful for moving lightly used, or background, apps from your main desktop. For instance, the iPad can be a great place to put a Twitter client that doesn't need much attention, or the palettes that clutter programs like Photoshop and Word (this is especially useful if your main computer is a laptop, where screen real estate is limited).
Not a Flash Workaround
The more clever readers out there may be thinking, "Aha! Finally a way to get Flash to play on the iOS." It's a good thought: since you're displaying your desktop programs on the iPad, and Flash runs on your desktop, shouldn't it run on the iPad under Air Display, too?
The answer turns out be yes and no. Technically you can display Flash content on the iPad using Air Display, you probably won't want to.
Fire up a video at Hulu and drag it to your iPad running Air Display and you'll see why: the audio and video get out of sync very quickly, frames drop from the video making it laggy and jerky, the image gets pixelated, and the audio outputs to your computer, not the iPad, so you can forget watching Hulu on the iPad on the couch.
It would be great if Air Display could let us view Flash content on the iPad, but the app's not ready for that yet.
Not a Remote Desktop
The other important thing to understand is that Air Display isn't a remote desktop. This isn't a way to access everything on your computer from elsewhere in the house. It's a way to use whatever windows you've dragged to the iPad. This is an important distinction; it's more like an extension of part of your computer.
Which is OK. Air Display doesn't claim to be a way to access your computer remotely, but users looking for that ought to look elsewhere.
If you're going to use Air Display, you may also want to get a stand to display the iPad, rather than having to prop it up against your computer or something else on your desk.
The Bottom Line
Just because Air Display isn't some things doesn't mean it isn't a worthy app. It is. You can't expect it to magically solve some of the limitations of the iOS, and it's not perfect, but if you need extra space on your desktop, and have programs or user interface elements that you want to shunt out of your main workspace, Air Display is a good bet. And, at only $10, it's a good value to boot.