I've been experimenting with a gel pen with varying degrees of results and surprisingly, it has lasted me about a couple of months. Actually, two of them with different point sizes. The Pilot G-Tec pen, so far, has shown some really good results on my sketchbook that has'nt failed me.
This is what the pen looks like:
Sure, anyone could use the old school Rapidograph pen set with different line quality but the maintenance is time consuming if one is not careful. The ink would certainly appear darker and dry up quickly. Especially with the thick paper surface of the moleskine. And just to show an example of what I've done with the pens.
I think these pens are actually great for visual exercises and creating line texture to build form. But also in that sense, it adds a level of raw energy to the image. It would'nt come to a surprise to me if the pens actually find resistance on softer or fragile paper surfaces, even watercolor paper. In my experience, it's usually the hot or cold press surface paper or illustration board that does a better job in letting the ink flow.
All in all, I'm really satisfied with the pens but will continue to experiment more in the future.