MaxiVista, $9.99, is the simplest of the four apps to install and configure. The MaxiVista iPad app simply searches for a PC running its Windows software, and automatically attaches itself as a second display to the first PC it finds. This makes getting started easy and should work fine in a home environment. However, I found out it could be a problem at the office. I used MaxiVista in an office where, unbeknownst to me, a coworker had purchased and installed the MaxiVista iPad app, too. The Windows software was running on both of our PCs. However, his iPad was not running MaxiVista when I launched the app on my iPad--and my iPad paired with his desktop PC instead of mine. Fortunately, I recognized his desktop wallpaper and went over to let him know what had happened. MaxiVista lets you designate a static IP address for the target PC, but this is not a foolproof method. There was one other installation oddity: it notified me that I was running the Avast Firewall software on my PC when I wasn't.
On the plus side, MaxiVista seemed to have the fastest display refresh rate of the apps I tested.
Splashtop lists XDisplay as a free app, but the free version works for only 10 minutes at a time. The fully functioning version costs $4.99.
XDisplay's Windows-side software, Splashtop Streamer for PC, also happens to work with Splashtop Remote Desktop for iPad, a $4.99 app that lets you control your PC from your iPad.
Like DisplayLink, XDisplay requires password authentication before allowing the iPad to serve as a second display. But unlike DisplayLink, XDisplay works with both Macs and PCs. Also, XDisplay's Windows software can be turned off when not in use. For all these reasons, I would choose the $4.99 XDisplay over DisplayLink, which is free, except for one thing. XDisplay had performance problems on my two test PCs. The cursor and application window movement was much too slow to be useful.
I should note that XDisplay 18.104.22.168, the version I tested, has an average customer rating of 4.5 stars out of a possible 5 with eight ratings. All of the app's versions have an overall rating of 3.5 stars. So, the slowness I experienced might be peculiar to the two PCs I used for testing.
For the office: DisplayLink
Based on my casual tests, the two most expensive apps--Air Display and MaxiVista, each $9.99--would be fine for home users. They're easy to set up and use and don't require password authentication. Despite the problems I had with XDisplay, hundreds of other users appear satisfied with its performance. You can download the free version and test it in 10 minute increments to see if it performs better on your PC.
But my pick? I used DisplayLink to write this review on my iPad as a second display. Its combination of price--free--password authentication, and acceptable performance suited me just fine.