An example shot of the efficacy of a grad ND filter (gradutated neutral density filter).
As you can see, the bottom of the filter is clear, the same exposure as my foreground grasses and trees. Towards the middle of the filter (graduated), it darkens to a (neutral) gray. The lovely blue sky peeking out is what I wanted, but the foreground is perpetually shadowed by these giant cliffs. Unfortunately, the filter darkens in a straight line and the notch does not. In darkening the sky, it also darkens the lower cliffs to near black, losing all their wonderful color. Now that I am better at combining exposures in PS, I would take the shot with the filter in place to ideally expose the sky and upper cliffs and take a second shot to expose for the middle red cliffs, layering them in to the final shot for the best of both worlds.
I took this both to show the power and the limitations of the grad ND, and find it very effective at doing just that.
Here endeth my Photo Tip o' the Day. :)