They're a little tricky at first try, and really thats all I've had time for with so much work on. Here's a very quick doodle just to test how the pens actually look:
The brush tips are smallish (the same as the black Faber Castell ink pens), and dont seem to be made for broad strokes, so it was an odd feeling shading with such a small tip. The ink is also translucent, so when you overlap two shades, it will produce a darker one (as you can see in the middle of the forehead) and because the shade is so clean, I can imagine it being hard to blend and create gradients.
I'll post something more once I've had a bit of practice.