Since I last wrote about Duet Display, about a year ago, it has received several updates. It can use your Mac graphics card to speed up rendering and it can be used wirelessly with an annual subscription. You can use both iPads and iPhones as extra displays for your computer.
In addition to working as an extra display, Duet also makes the extra screen touch controlled. It is like using your fingers as mouse pointers on a computer.
If you have a headless (a computer without a display) Mac, you can use your iPhone or iPad as it's only display. I have done that with a Mac mini.
Duet Display has a 4.2 rating from more than 300 users in App Store.