Works quite well. The method of fixing it to the camera is a bit fidley - you have to buy screw an bolt (tiny size) separately- and although it doesn't fit 100% to the contours of the camera it sits quite sturdy. I bought a Caruba alternative for the Nikon DK-19 eyecup and that fits quite tight round the adapter by the elastic tension of the rubber eyecup. I'm not entirely sure whether it will stay on after prolonged use. And it does block the screen/viewer switching sensor
I bought it because I'm using my left eye and my nose sat against the screen. The extension helps a lot and it blocks out stray light quite efficiently.
A bit costly in total but I can recommend it.