I believe that the type of pen or pencil used for sketching and drawing is such a personal choice; I have gotten into heated discussions with artists on which is the best and why. I used to love Pigma Micron pens, the nib was so fine and the ink was waterproof and I loved how they flowed on the paper or fabric. But soon I found out that the ink has a tendency to dry out more quickly than other pens. I tried all the recommended fixes, store the pen on its side, shake it before use, etc. but still they would be "unresponsive" after what seemed like a very short period of time.
A friend recommended the Faber-Castell Pitt artist pens. I tried the superfine one in black and loved it - they are so smooth and don't skip or scratch. And they last! They also come in sepia which is a luscious brown (and the one I used in the above sketches). Today I found that F-C has released a new nib but only in black, Extra Fine nib.The pens are waterproof so that you can use watercolor or gouache over a drawing with no problem. Faber-Castell also has brush and big brush pens in wonderful colors and warm and cool grays.
Some people swear by the Rotring Rapidograph but I found that they leaked. Others favor the Staedtler pigment liner. In the long run, though, I guess what matters is what works for you. I tend to find something I like and stick with it until usually the manufacturer discontinues it. Let's hope that doesn't happen at Faber-Castell!