This review is by Kalina Wilson, who can also be found at geminica.com.
The brushes on the Kuretake Disposable Pocket Double-Sided Brush Pen are felt tips that are designed to give a variable width line. These brushes are in the same neighborhood as Tombow Fudenosuke brush pens (of which I am a great fan) in terms of line width, shape, and responsiveness.
The two tips are described as "fine" and "medium", but with a light touch, they can both achieve quite a fine line.
Neither of this pen's tips are intended for fast, lush sketching. Ink flow is quite thin. You get a lot of control but not a lot of ink. I'd say this is a good pen for cartoonists that are inking carefully over pencil sketches - not so good for loose sketching since you'll run out of ink if you move too fast. The ink appears to be totally waterproof.
This pen would be a solid competitor to the Tombow Fudenosuke... if the "medium" brush were a little more reliable. Mine hasn't been used much but it's already a little flakey, and its skimpier ink flow really becomes obvious if you try to fill an area - see the bottom of sketch on the right.
Because this pen is double-sided, affordable, waterproof, and has at least one good fine brush tip on it that can achieve very good detail (that Spock drawing is quite small), it's a very decent pen to experiment with - but I would recommend giving the excellent Tombow Fudenosuke a try as well.