The Akashiya Sai ThinLine Brush Pen Extra Fine, Sumi Black just sucks. It has no redeeming qualities: it's way too dry, it's ink capacity is low, it's tip turns into a ragged mess quite quickly. The only good thing about it is that you only wasted about 5$.
I saw Kim Jung Gi using these things for a while a few years back in some of his work, and possibly production has changed or I got a bum one, but I can't see how he did. This thing is the absolute driest brush pen I've ever used. It is very prohibitive in how you use it, as I can barely make a mark at any normal pace, thus most likely really stiffening up anything you ink with it. It's dry brush marks aren't even very good looking. I can't imagine using this brush pen on a watercolor type paper either.
Drawing with it feels like a chore. The ink isn't very black at all, and also it is a closed pen, so you can't refill it with cartridges or bottled ink. It is water proof, though if you rub on it while wet the top layer will lift a tad (though must brush pen ink will do this). The bristle tip makes these ragged marks that are not consistent. There is no bounce to the bristles. It's just all ugly here.
The tip also does not hold a point in the slightest. It will fray out and stay that way until you spend a few moments reshaping it, and this happens quite quickly after you start drawing.
Unless you can't help yourself, or you think that the dryness of the Akashiya Sai ThinLine Brush Pen will be something you like, just don't get it. It's awful. 1 out of 5 Zaffino Crazy Face Franks here.