I'm in the same boat. There are different metrics to judge the quality of an upscale (peak signal to noise ratio comes to mind), but it's obvious they are limited in that they can't capture perceptual quality very closely. While it's obvious the filter is much sharper than even NGU sharp, it also seems to come with a weird gradient effect and some artifacting. Another thing I find is that sharper filters like NGU sharp don't upscale as well as other upscaling options on frames with edges that aren't supposed to be sharp, probably because in some sense they try to hard ink in parts that aren't supposed to be so. This can happen either because the source is composited to be blurry for artistic effect, or because the source is low quality. I admittedly have not tried Anime4K, but I imagine Anime4K will have a similar effect.

Of course, in the end, it's an entirely subjective thing. Personally I hold off using on using NGU sharp and use NGU Anti-Alias instead for the above reasons.

EDIT: this is addressed in the readme: I think the results are worse! -Surely some people like sharper edges, some like softer ones. Do try it yourself on a few anime before reaching a definite conclusion. People tend to prefer sharper edges. Also, seeing the comparisons on a 1080p screen is not representative of the final results on a 4K screen, the pixel density and sharpness of the final image is simply not comparable.

EDIT: I just tried this filter on a 4k monitor. To honest I don't think is very good. To me it reminds me of the bad parts of sharpeners turned up to the max. All the edges turn into a weird, sometimes jagged, smear, and originally blurry but detailed backgrounds just become a weird mess. I really don't think even people who like sharpness will prefer this filter for general viewing, and I find the chart given in the preprint (https://raw.githubusercontent.com/bloc97/Anime4K/master/resu...) extremely dubious.