I had a go on one of these last night. One of the lads brought it over to show me while I was doing a borescope on a 737.
As you'd expect from £20 optics, the picture quality is only just bearable.
It focuses pretty well up close (3-4"), any further than 6" and the picture is pretty poor.
Obviously there's no control over where it goes.
Pretty gimmicky overall, but it could help you find that dropped washer or read a part-number without removing cowlings.