I've got one as well. The image circle is large enough for full frame and it makes the bokeh & aberration rich images similar to other Rodenstock lenses though it should be noted that when the lens is held flush to the Nikon F mount maximum focus distance is somewhere in the neighborhood of 4 inches/10 cm from the front of the lens. Very short working distance.
That said you may be able to remove the rear lens group and get some more working distance out of it, I've had success doing this with other fast 42mm lenses, haven't tried it with the Heligon 42mm yet.
I bought mine several years ago for $50-$75 if I remember correctly. They are listed on ebay quite often for $200-$300 which I think is certainly over priced considering the very short working distance.