Heliopan Variable ND
Variable ND's are all about speed and scale.
If these factors were not an issue I would always utilise regular fixed hot mirror ND's with the occasional Pola combination. This will produce the best optical results every time.
But if you need to go fast and lightweight the Variable ND is a good compromise.
Until recently too much of a compromise to my taste and I would certainly not have considered using one for cameras with resolutions above HD... but then I have been very impressed by the Heliopan.
It offers speedy light control just shy of 7 stops. Its German build quality is solid with marked increments and an end stop. The glass (Schott/Zeiss) yeilds very sharp results... better than all I have seen.
I would primarily use it with stills lenses to achieve a more run-and-gun configuration.
Here with a Red Scarlet and a set of cinevised Nikkors.