It looks like an interesting product but I don't think it could match the resolution of your 500 f/4 lens (which would have a objective diameter of 125 mm vs 90 mm). Additionally, it won't have autofocus or variable aperture. So I suspect for many photographic-minded people, given the substantial cost, it will be better to save up for the additional money needed to purchase a "regular" supertelephoto.
Incidentally, I wish some someone would make a simple image erector for standard telephoto lenses. Nikon made one many years ago and from what I hear it gave great images. I imagine it would rather simple to construct a basic image erector for telephoto lenses which could accept basic astronomical eyepieces and give great images. Such a device should be relatively cheap to produce.
