My guess is that you'll most likely to be shooting with the assistance of a tripod whilst using GND filters.
If so, handling / positioning the filters is a piece of cake since you've got both hands free to do it.
The filters are plastic and rather prone to scratches.
Nevertheless, the scratches/dust will hardly, if ever, show up on your photos due to the filter being positioned within your lens' closest focusing distance and the fact that you'll stop the lens down most of the time. Just replace the filters if they get too banged up. Only @ S$20-30 a piece.
Most manufacturers have filters in either 'P' size (84mm width) or 'Z' size (100mm width), so it will be advisable to buy the filter holder in such sizes. Use 'Z' size if using really wide angle lenses (< 20mm, 35mm equivalent). Bigger filters cost more obviously. Get only the biggest size that you need, save the money to buy more filters.
Some tips, which you may already know: -
1) Meter with the dark portion of the filter covering your lens. The rating of the ND may not be accurate. A ND rated at 3-stops may be only be worth 2-1/2 stops. TTL metering eliminates this problem.
2) Effect more pronounced when lens is stopped down. that's why you're most likely to be using a tripod. and that's why the filters should be quite easy to handle.
3) Graduation effect / line can be quite hard to see. If lens aperture is electronically controlled, use DOF button to stop the lens down (also see point 2). Of course, ALWAYS position the filter by looking thru' the viewfinder...
