the best one i've used is the singh-ray. expensive but head to head is better than the tiffen, which i also have, and both are better than the lightcraft workshop variable nd.
out of all three, for best quality i'd use singh-ray, but best value is tiffen.
I think the 31 mph smart mode in the article is actually the sport mode
the colors look GREAT! downloading the sample footage now. will be picking up one or two GH5s for sure
damn fantastic flying