|I have an eteq.
Its not bad, but you need an additional source of light. I have found it very useful for a few things but not very useful for others.
The bad - not very good for looking inside the cylinders and the mirror to look up at the valves is useless. The light is not very bright so an extra light source is very helpful. Takes a second to connect to the iPad which is annoying sometimes.
The good - great for everywhere else, inside the wing, fuse etc. nice to be able to take video/pics and have them be just normally part of the iPad photo / video library. The fact the display can be remote allows you to do things you can't do with other tools. For example you can watch a pulley or cable while you move a control stick 10 feet away.
Found one really nice use during a correlations check. You can point it at the ASI and ALT then from outside the plane while working the test equipment you can see the instrument response. Makes a two person job into a one person job. Can also use it to adjust the encoder screws and see the altimeter.
These things will get better and better and I imagine a 4K version with a top of the line iPad would be pretty decent but still the mirrors suck so looking at the valve faces and combustion chamber is hard if not impossible at least with what I got.
Certainly was worth the relatively cheap price I paid.