Hi, first of all, let me introduce myself, as I am new to the forum. I am a physicist with an inclination towards other more artistic activities. Due to some unfortunate encounters with fountain pens (not good quality, leaky samples) I disregarded them for some time. However, drawing and design has brought me back to them as a very powerful medium to capture my thoughts and, even now, to also work. What follows is an exceprt of a personal log that I keep to myself, where I will be testing some pens I ordered http://juanjose.garc...drawing-and-fix While I know some reviews here, they were not complete enough for me to make my own mind, which is why I am redoing them from personal experience. I hope this is not a problem.
The Tachikawa School G is a relatively inexpensive pen that looks like a disposable rollerball in its plastic design. It comes with one standard size cartridge that contains a waterproof ink which does seem quite thick. The pen is advertised as useful for Manga, which seems like a good start. I ordered via Amazon and got it one week later in a nice package with two extra cartridges, all together under $20 shipped to Spain from Japan by Amazon
Initial fixes
The pen looks cheap'o, and indeed it is, but it also feels good on my small hands, which are used to Pilot's and other cheap writing devices. I plugged the cartridge into the nib and started scribbling. The result was thin, straight and consistent lines, very similar in color and precision to those of a 0.2 mechanical pencil. I was astonished by that, but after one page of test strokes and hatching, which seemed too thin and too volatile to be true, the ink stopped running.
Some research revealed some problems: The point was scratching the paper, which was of moderate quality but not problematic for other pens. The scrathing caused threads to enter the nib, mix with the ink (which dries super-fast) and clog it. Dripping the pen in water helped momentarily, revealing what should be the actual linewidth of the pen, but it stopped working again for the same reasons.
Fortunately for us the internet is full of solutions. One is this: take a simple grinding paper, very fine one, such as 2000, and cut a small piece. Drip the paper in some water and start writing circles with the pen. Write a couple of circles or eights; go back to a normal paper and test. Again and again, always softly, always keeping the grinding paper soft and trying different angles. This smooths the pen, restoring it to its ideal state, and removing imperfections that were not detected in the fabrication process or quality assurance control (after all it is a very cheap pen). Some people will argue that 2000 is to coarse, but that's all I have and I routinely use it to polish my nails before playing guitar, so it is really not that bad. In any case, if you are worried, you can wear off the paper first by scratching one piece of paper against another until it feels smoother.

Finally working
Fixing the nib as explained above was not traumatic at all for the pen. The nib itself is very fine and quality controls for such a cheap pen cannot be that good: I just got a bad sample. Once the nib was smoothed, however, I got a very nice working pen that produces the right kind of line for my purposes.
Below I show some line samples of the Tachikawa School G, a rollerbal pen from the inexpensive brand Muji, and my loved Rotring ArtPen EF, which I have been using for the past month. The nib drawing shows the direction of the pen when sketching and the "inverted" sample means that we put the pen upside down and write with the back of the nib to get a finer line.

As seen in this picture, the line width of the Tachikawa pen seems to vary between 0.8 and something like 0.3, depending on how we press and orient the nib. This is great for drawing, though a bit more flexing would be even more useful.
The ultra-fine marks when drawing upside down are as fine as a 0.2 mechanical pencil, and you may even get them finer by pressing less. This is good for fine detail, early sketches and creating softer shadings.
Apart from the pen, one should pay attention to the ink. The different flow of the ink in the paper creates marked differences between the School-G and the Rotring, which you can seen through the "feathering" of the horizontal lines in this medium-grain paper of the Rotring; not so much feathering in the School G, which looks sharper.
The School G's ink is waterproof as advertised, provided that you wait enough for it to dry. I have tested it by smudging the paper later on with a pencil and some water. Compare the effect in the Rotring ink (which I love for shading drawings) with that of the School G. The fact that the lines remain sharp and clear is a plus for using this pen in sketches that also involve watercolors.
Conclusion
Overall, I am now happy with this tool and hope to see how it evolves. The finer line is very useful, as I had been currently limited by the Rotring's ArtPen darker tones in what I could draw, even if they are only simple learning exercies for hatching, shading, etc.
On the other hand, to be fair, while the pen is fine, both in the literal and in the figurative sense, somehow the ink does not cooperate well with it, which is the biggest annoyance. I keep it in my drawing kit and I am combining it with the Rotring, but I have to always remember to test the pen in a separate paper to see whether it needs to be restarted, either dipping the nib in some water and initial sketches in trash paper.