I recently purchased a second hand ND8 (ND stands for neutral density, and the number denotes the strength of the filter) 67mm filter for my 18-70mm lens for the sole purpose of getting long daytime exposures. The ND8 filter cuts out 3-stops of light, allowing you to use a longer shutter speed. In most cases, people use this in landscape photography to get blurred motion in water to get that painterly effect. Others use it for when they need a certain aperture value for creative depth of field.
Today was the first day I actually had a chance to use the filter. Essentially, it does what it says on the box. I wanted to see how long I could drag the shutter in the daytime so I reduced the aperture to the minimum (F/22) and stacked the ND8 over a circular polariser.
Circular polariser - wide angle (F/22, 0.6s)
Circular polariser + ND8 - wide angle (F/22, 3s)
Circular polariser + ND8 - zoomed in (F/22, 2s)
With the stacked filters, the air bubbles on the surface just melt away forming smooth white streaks. Stacking like this will definitely make waterfalls have that dreamy look. Only draw back is the heavy vignetting at wide angle which disappears when you zoom in (the dust in the third image due to the small F/22 aperture... ** note to self, make sure lenses are clean!!). However, I intend to use it differently and will share that with you once I've had a chance to experiment again.
Also, to better understand the actual effects of a single ND filter, I will need to do another back to back comparison between a filtered lens and an unfiltered one to see if there's any perceptible difference in the final image in term of colour or contrast.