My understanding is that they're intended for D!g!tal SLR users making HD videos. It allows them to adjust exposure quickly while zooming, changing environment, changing aperture to adjust depth of field, without having to change iso. Purely anecdotally, I've heard the cheap ones have erratic light transmission issues such as banding and vignetting.
"There is a time and place for all things, the difficulty is to use them only in their proper time and places." -- Robert Henri