PROS
-good optical quality
-hard stops at either end (avoiding the X-pattern)
-easy to read ND values on the edge (in stops)
-lightweight
-fits well on the camera (fits over the original lens, so it does not affect the camera’s waterproofing)
-quite good value

CONS
-the “knurls” on the outer ring are tiny, it would have been far better to have more substantial knurls that stick out 90 degrees to the lens axis, to make it far easier to turn the filter. As they are, I have to hold tightly enough to turn the filter, but then a bit of my thumb and finger curls around to leave fingerprints on the periphery of the front of the filter.
-I noticed a tiny colour shift as I turn through the range, but this is easily dealt with in post
-pushing the filter on to the camera lens tends to result in fingerprints on the filter. Best to use some microfibre cloth between fingers and filter

NOTES
-I shoot a lot of timelapse with my Action 5 Pro, and having a very dark ND on it is necessary to lengthen shutter speeds a useful amount (I’ll often shoot at 1/8 to 1/2 second when shooting crowds, and up to 2 seconds when shooting skies, to blur out birds). A dark vari-ND like this one is perfect for my needs (far more convenient than a series of ND filters)
-I don’t tend to take my camera underwater, but if I did, it looks like this filter would collect water between lens and filter. It’s a dry (or perhaps rainy) filter only

MY STAR RATINGS:
1* substandard, mostly cons
2** barely acceptable, cons outweigh the pros
3*** meets the basic requirements, cons balance the pros
4**** good product, pros outweigh the cons
5***** exceptional, all (or almost all) pros