It makes the viewfinder as big as my F100's
The only niggle I have would be that the extreme corners of the frame may vignette a little depending on how close your eye is to the viewfinder piece. Also the ISO data on the right side of the frame can be blocked, and only revealed if you move your eye a bit to the left. But that is not important to me because I check the ISO on the back of the camera anyway. I'm gonna give it a try this weekend on two upcoming weddings.