Delivered and tested today. I'll spare you the brickwall shooting, which tells what everybody already knows: some wavy distortion, significant vignetting wide open, stop down to f/4 to get acceptable corners, etc.
Otherwise it is a fine lens and, for the price, unbeatable. A used Nikkor 20/2.8 would bring about the same quality and flaws for about the same price. My first impression is that the Viltrox is cleaner (lower CA).
The first pic captured the whole work of art but required keystoning correction. The 24/2.8 PC would also have done the job, in a different way.
The third pic was shot in a pretty dark church (Lille, St Maurice) by gloomy weather . At f/4, 1/4s, and 4000 ISO, IBIS made it possible if not perfect.