A few simple shots with a very cheap variable ND filter made by Neewer (just $14 for the 77mm). Shot on a Sigma 17-50 OS, this is just a sharpness test, not a colour-shifting test. I saw no moire with this ND, though, and colour shifting seemed to be minimal. It's up to you to decide if the softening is acceptable.
The basic summary is that the ND will often soften a shot, but it can be mostly compensated for by Unsharp Mask. The video shows with ND, without ND, and with ND and sharpening. Download the original ProRes HQ file (over 1GB) if you want to pixel peep. No colour grading applied, so it's flat. Shot in ProRes HQ with Film dynamic range.
Note that there's a Sharpen effect, but no Unsharp Mask effect, built in to FCP X, so you'll have to make your own in Motion. Applied here with Radius of 2.0 and Amount of 1.0.