I explain why ND Grads are good

When you're doing landscapes you'll notice that the sky is always, ALWAYS brighter than the ground. Now, each camera has something called a 'dynamic range' - this is basically the amount of stops between dark and light that the camera can record. If you've got a budget camera then the dynamic range will be smaller than a really good camera which would have a larger dynamic range.

What does this mean? Well, you or your camera will meter for the conditions it sees. If you're using spot metering then if you meter off a dark area then the dynamic range thing means that the lighter areas are likely to blow out (not contain any information). If you meter off a light area then you may find that your dark areas contain no data whatsoever. The normal evaluative metering tries to get an average of the whole scene - this may mean your light areas blow out, and your dark areas contain no detail - but it's usually the best compromise.

So - practical example for you to try. Tomorrow, get your camera and put it in spot metering mode. ISO 100, f/8 and aperture priority. Aim at the sky and see what your shutter speed will be... I reckon quite high. Now, without changing any settings, aim at the ground and see what the shutter speed is. It WILL be much slower.

Right - so getting back to why you need grad filters. Basically, what you need to aim for is to get the whole scene within the dynamic range of your camera. The easiest way to do this is by darkening the sky. By using an ND grad, you slide it down in a holder in front of your lens, until the sky part is darkened to your satisfaction - you're aiming to get the sky and ground to around the same brightness so that your sensor can cope. Because grads only have the ND covering over half the filter (please don't bother with soft grads) it lets you darken the sky and bring it back into the dynamic range of your sensor - therefore you don't get blown out areas (or at least much fewer blown out areas), and the ground is exposed nicely and not too dark.

With solid ND's you're not really doing much except darkening the whole scene uniformly. Fine if all you want is to lengthen the exposure to create milky water, moving clouds etc., but to keep the detail in the sky and ground when doing sunrises / sunsets you'll be after a grad.