They're worth it if you are using an in camera meter. Note that you can also make your own by using two old linear polarizers stacked on top of each other.
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.