Hi Fintan,
I own the kenko counterpart, I think they are pretty similar, and I'm very happy with what I'm getting, taking in consideration the huge price of the original Zeiss Mutar. Obviously, I don't print very big, but I can't notice any loss of image quality (sharpness, contrast, etc) by 25x25 cm print size. I've tried it with my 80 and 150mm CF's.
You lose 2 stops, so compensate accordingly.
Be careful assembling the set, follow a simple formula.
* Wind the camera
* Take off the lens
* Attach the converter to the body
* Attach the lens to the converter
* Shoot (and remember to compensate those 2 stops)
To unmount, reverse the above procedure.
You should check the screw of the converter before using, the head should be horizontal and not at 45 degrees angle, use a screw driver gently in this case and turn it clockwise until clicks and lock, now your converter is also cocked.
Good luck and have fun!
Cheers
André