ND filter is something that most of the landscape photographers love to use. There are different kinds of filters and few most widely used are the ND filter, hard and soft Neutral Density filters. You basically need to know when you need to use and when not to use these filters.
I have seen few people using these filters where it should not be used. When it comes to a landscape shot full of mountain peaks, snow covered peaks, tall structures etc, hard ND filter can cause a sudden change in contrast and brightness of the image at the center of the image. This is because the upper half of the filter will have one or three stops whereas the lower half of the filter is pure transparent glass.
In hard Grad ND filters, the transition is a hard or sudden change in stops whereas in soft Grad ND filters the transition is smooth. I would recommend using a soft ND Grad filter for smooth effect or an ND filter which has no transition in stops and is the same throughout.
Here is an example of where I have used ND filter and is should not be used. As you can see the mountain peak is dark, but I just wanted the sky to be dark and create some long exposure shot. You can clearly see the transition. In this case, a polarizer along with ND filter will be helpful. This kind of filters is good when you have different layers in the landscape. For example, sunset where the sea and sky have a clear line separating the two.
There are very good filters available in the market. Lee, Haida etc are some of the filters available at my place but very expensive. I use Haida 150*150 10 stop ND filter for my Nikkor 14-24 mm f2.8 lens. You need to buy the holder for this filter separately which will cost almost the same as the filter itself. These filters are huge and hard to carry it along while trekking, as I do a lot of trekking in the Himalayas.