Where things can get complicated is in the choice of filters, particularly with graduated neutral density filters (which is a bit of a mouthful so let's call them grads for short). For those who aren't familiar with them, grads aren't complicated, they are filters which are half clear and half tinted with a neutral tint. The idea being that in situations when the sky is a lot brighter than the foreground (sunrise or sunset for example, when the difference in brightness is often greater than the camera can capture) the filter is positioned in front of the lens so the dark half covers the sky with the clear part over the foreground thus balancing the difference between the two to a level that the camera's limited dynamic range can cope with.
That's fairly simple, so what's the problem? Well, 'which graduated filter should I get' is not a particularly easy question to answer as the choice is dependent on what you will be photographing and crucially how much you want to spend.
Like most filters, ND graduated filters come in various strength tints but they also come in different graduations, usually soft or hard. Soft being a gentle graduation between the tinted and clear areas while hard is a more sudden change. The problem has always been that there is no 'one size fits all' graduated filter... Hard grads are suited to scenes where the horizon is clearly defined, the coast for example, where it's easy to position the filter along the horizon, while soft grads are ideal for situations like mountainous landscapes where there is no clear horizon and the soft graduation blends seamlessly into the mountains. If you take soft grads to the coast however, the soft transition won't be strong enough to cope with bright areas near the horizon, conversely using hard grads for a mountainous scene will leave an unnatural dark line across the mountains. So, when somebody asks which the best single filter is to start with, or something along those lines, I have to go into a long and windy explanation of the strengths and weaknesses of each.