I discovered the G-Tec-C at Muse Art & Design, which is a really sweet local art supply store here in Portland, Oregon. Since then I've picked up Hi-Tec-C pens from both Jetpens.com and Kinokuniya Japanese stationery stores (which, in Portland and Seattle, are attached to Uwajimaya Asian groceries). The great advantage of the G-Tec-C and the Hi-Tec-C is that they have a very fine point that moves smoothly over toothy paper, such as the thick Aquabee Super Deluxe Sketch Book paper. In fact, they are so fine that they are easy to use for sketching - you can easily ignore bad lines by just reinforcing the good ones.
These sketches were all done with G-Tec-C:
The Hi-Tec-C is very similar to G-Tec-C, except (a) they come in more colors and (b) supposedly they are not water soluble. I find that they are quite water soluble, though how much does tend to vary depending on color, type of paper, and age of drawing. Here are a few spots where I used water to spread color in my Hi-Tec-C drawings immediately after they were made:
I've been enjoying using these pens as a way to get double the work out of each sketchbook page. Long after I'm bored of the sketches I did, I can go in with Hi-Tec-C and add subtle shadings, backgrounds, or just simple coloration. It makes the page a lot more fun to look at, and I get more practice out of each sheet of paper.
The pens also come in some great colors. There's a Japanese series called "yawaragi" which includes Fuji Reddish Wisteria and Cherry Blossom Pink, European colors that include Yellow Ochre and Pompadour, Nature Colors, and Cosmetic Colors.
Here's my set as of today, with samples of the line produced by each:
These pens are so much fun to work with! Highly recommended for anyone that likes sketching with fine lines, or just wants a way to add fine lines to existing drawings on watercolor paper where the pigma micron can't handle the paper texture.