I have tested the external viewfinders for 25/28 and the 21. I find it quite easy to see the framelines, but due to the elongated distance of the eye from the exit pupil of the viewfinder, the framelines are visible only on a black background and not the actual viewfinder image. At least, that is true for me.
With a little extrapolation and by using your judgement, you can, however, quite well estimate where the image will end. Nevertheless, I have not done a careful comparison between developed image and the actual view through the finder, whether this assumption is true.
Also, it takes some time to get accustomed to the correct position from which the complete frame is visible. Nevertheless, the rubber ring protecting the entrance pupil is very nice if you wear glasses: you won't get any scratches.