I wasted some money on Mini Display because some people I usually respect said it was a good buy. It is a terrible buy. Don't buy it!
First, why would I (or anyone) want such a thing? It lets you use an iPad as an external monitor. That's useful to me because I like really little computers and also often have a need to see two things at once: a reference list while I'm writing something, say, or a list of acronyms for an article I'm writing. Sometimes I just like to have music running on the iPad next to me (on its Book Arc) so I can see what's playing. I pay for Spotify, so I could just run it on the iPad, but you get the idea. You could also let Twiter scroll on the iPad or whatever.
I bought Mini Display with some misgivings, including its $12.99 price, but it's from the makers of Screens, so I figured it was probably badass. It also had some features like being Retina ready and being universal that I thought might be useful or nice.
It lags to the point it's not usable. If you touch the iPad with your finger or use the mouse, either way, be prepared to wait a few seconds for anything to happen. It works, I suppose, because the desktop appears on it. The creators are getting a lot of mileage out of how easy it is to make it work.
After quickly becoming disgusted with it, I remembered doing this before, a long time ago. I dug through the really large list of iOS apps I've purchased over the years and came up with DisplayPad, which is cheap ($2.99) and works better. I wish I hadn't wasted money on Mini Display, but perhaps the spoils will allow the developers to make it fabulous and then I'll be glad I have it.
They both work the same way. You download a companion app to your Mac and open the app on the iPad and pick to connect them. They're both childishly simple and this part works fine. You adjust the displays just as you would with any external monitor.
DisplayPad doesn't lag at all. It isn't a Retina, pretty display, but you can pick to change the display quality with both apps. Changing it with Mini Display makes no difference in lag. Leaving it on defaults with DisplayPad makes it incredibly handy. Both allow you to flip the iPad all around and they adjust the screen to appear upright, but Mini Display left me with a plain blue screen that didn't recover a couple of times.
If you've never thought of using your iPad as an external display, give it a think. It's a good idea for certain things if, you know, you don't already HAVE an extra display. And save your money. Use DisplayPad.