I have Cokin and Hitech filter holders and ND Grad filters in both P and Z-Pro sizes. I also have the LEE SW-150 for the 14-24mm f/2.8G AF-S Nikkor I am very happy with all of them. The Hitech holders are aluminum (not plastic as the Cokin) and take all brands of slide-in filters.
I have grad and full ND 4x6 filters. I tried to handhold them but didn't like the experience as I was always afraid of scratching them on the front ring of the lens. I didn't find comfortable and didn't get pleasure to handhold them.
I now have two filter holders that I keep installed on two different lenses when I'm shooting in one place (so, no need to uninstall and reinstall these when changing lenses). Typically, I use the filter holders on my Nikon 16-35 f/4 and Nikon 24-120 f/4. I also use my Samyang 14mm f/2.8 and Nikon 10.5 mm fisheye for landscapes but without filters. I rarely use my 70-200 for landscape, and when I do it is mostly without filters. So, two filter holders is good for me.
The two filter holders are Cokin Z-Pro and Lee System for 4x6. I first bought the Cokin Z-Pro as it is less expensive and does the job very well. A couple of years later, I bought the Lee system with the add-on to be able to rotate the second HD filter when I need two filters in specific scenes. At first, I felt the Lee system was less reliable because of the way the filter holders attach to the lens, but after getting used to it I now prefer the Lee over the Cokin. In particular, some Lee rings are especially suited to minimize vignetting for wide angle photos.
Of the times I use ND filters, 90% of the time I'm using graduated ND filters. I typically use a single slot P holder, but every now and then I use two filters or more. Last year I found myself using the second filter handheld at a different angle from the first to hold back both a rising sun and a valley with fog.
My use of ND filters is almost always for stream photos - so I use the Vari-N-Duo. I have also used a CP as an alternative. Occasionally I use a CP and multiple exposures in place of a ND filter.
I use grad filters in different strengths and gradations, all 4x6 Singh-Rays and usually handhold them, but sometimes use a Cokin holder with them. Even with increased dynamic range in the newer cameras and software options such as blends and HDR, I like using them.