I've finally bought myself an inkless pen just to see how it actually works. The one that I bought just cost less than US $10. You can probably find them cheap and in abundance on eBay. On Amazon, there are more fancy and expensive ones.
The metal tip is the screw on type. It's not particularly sharp.
That's how the metal tip looks like compared to a solid stick of (watersoluble) graphite. The tip does feel very hard.
The lines produced by the inkless pen look very much like those from a HB pencil.
You can shade using the sides but you won't get a large area for shading. You also can't blend the shading since this isn't exactly graphite. When I ran my finger over the area, my finger did not pick up much of what created the shade.
The lines are waterproof, just like normal graphite.
I'm glad I spent less than US $10, including shipping, for the inkless pen. It feels like a gimmick. You can get similar lines from a HB pencil. When writing, sometimes the tip would feel rough, and sometimes smooth. The part on the tip that wears out will be a bit smoother. Yes, the metal does wear out. Or maybe it isn't metal in the first place.
Anyway, since the lines look like graphite lines, it's probably more worthwhile to spend the US $10 to buy a good box of pencils.