Just started trying AK markers too, like Graham. I tried them:
- on dark grey plastic (e.g. Eduard) and achieved full coverage in the 3rd coat.
- on shiny, steel-coloured PE parts of seat harness: full coverage in the 2nd coat.
I have not tried them on resin and 3D printed parts yet.
A big question for me is how long will the painting last when I work with the PE parts (e.g. bending)?
The colour selection is relatively poor, even if you buy this full box of 34 colours:

The expected advantage is the time saving when painting small parts. Painting small parts with airbrush is an extremely time-consuming process, mainly not the painting process itself, but the additional work (dilution and mixing, color change, cleaning the airbrush).