I tried this on my M6 and it magnifies the view so the framed image is approximately the size of the next larger frame; 90mm appears like 50mm and 135mm appears like 90mm. It does not distort; the framing is correct, it is just that the overall image is larger. It is not useful with 35mm or 28mm unless you really press your eye to the camera but for long lenses it is a lot easier to compose and focus.
This unit is in mint condition in the original case with a handy leash. The glass is clean and scratch-free.