I have a couple of old ones, found over the years, one of which I adapted for the HP finder with a little home made thread adapter, more for the exercise than the need. If you're doing micrography or closeup work at an odd angle, it's a good thing to have, and the diopter adjustment is handy. But because the horizon shifts when it's rotated, and the image is smaller than what you usually see, it's no fun to use if you don't need it, and I would not pay real money unless nothing else works for you.
It's telling that both the ones I have were gotten for almost nothing, mint in their boxes. They don't get much use.