Two main problems:
1. If you use the retina resolution for the iPad the letters are extremely small and you basically need a magnifier or vulture eyes
2. If you turn that checkbox off (i.e. no retina display), you get a more acceptable resolution however, the letters are fuzzy and there is something going on on the display. I bought the app to read web help displayed in FF for a program I am using. As I hover the mouse in the page there is flashing on the screen. It is a bit hard to describe but it is annoying (the effect looks a bit like the old CRT terminals).
I tried the HiDPI resolution but it doesnt have any noticeable effect as far as I could observe.
B0gdan about Air Display 2, v2.0.4