I borrowed a "cheapie" off a buddy, he picked it up out of a hock shop! (his favourite shopping place! - LOL)
The model was pretty basic, and the resolution was only moderate - and the screen was too small (about 3" I STR). I used it to examine the lower part of the cylinders and pistons in my Cat 931B traxcavator (3204, 4 cyl engine).
I had to chop the rusted sump part off the oil pan, and as I found a couple of pieces of piston skirt in the sump, I had to try and examine the underside of the pistons.
So I shoved the camera and wand up to the rear of the engine and tried to get it up the underside of the bore - but I only had a moderate degree of success.
It was difficult to see what I was looking at, as the camera field of view was quite narrow - and trying to reposition the camera lens location, with a twisty, snakey piece of wand proved to be difficult, to say the least.
However, I did get some idea of what was there - enough to reassure me that the piston wasn't falling apart completely, and that I could start it and run it.
I'd like to buy an inspection camera, but I'd like to acquire a unit with a decent field of view, and also have a bigger screen to view the images. The phone app arrangement sounds like the way to go.