Cokin ND Graduated Filter Kit P Series, with Filter Holder & Graduated ND Filters #121L, 121M, 121S)

I don't know its behavior with film, but with digital cameras, don't put 2 or more Cokin ND Graduated filters, one above the other. The magenta cast is awful and don't try to correct it by modifiying your in camera's color temperature: it will affect the non-filtered area. If one filter isn't enough, try the old and true Photoshop trick: with the camera in a tripod (ideally), take two -or more RAW shots, one for the highlights and one for the shadows (if I have no tripod handy, I use the bracketing and high speed in my camera to take a couple of pictures, trying not to move); then mix them in Photoshop and.... voila!!!! No need to bother with rings, attachments and delicate and easily scratched plastic filters.

To obtain well exposed landscape photos when the sky is much brighter than the midtones and shadows, graduated neutral density filters are a must. These work exceptionally well. They slide up and down so you can meet the horizon line. They also rotate vertically. I use a 2 stop ND filter in all my landscape shots with anything other than blue skys.

I really like these ND filters. Unlike a screw-on filter these can be adjusted up or down depending on where the brightest part of your scene is going to be. If you have the multi-holder, you can also stack them to increase the effect.

Using ND filters takes less time and effort than stacking exposures in Photoshop... the only downside is that ND filters like these have a stright transition, so if you have a very jagged skyline then hit can artifically darken some peaks in an image.