I don't have the accessory lens(es), and they seems to be very hard to get. According to Hermann in this thread, they work quite well. He's referring to the Zeiss Turmon, not the Docter/Noblex but I doubt the optics have changed apart from coatings.
https://www.birdforum.net/showthread.php?t=385172
The Minox and Leica (with close-up lens) are equivalent to a 8x loupe with a 20-30cm standoff distance, or a 5x loupe up close.
The Nikons dont't really have that level of close-up ability. Good enough to view details in the Mona Lisa, but not to read the fine print on a pill bottle.