The Le Pen set I have includes a 0.8, 0.5, 0.3, 0.1 and 0.03, and one brush tip pen. I use the 0.3, 0.1 and the 0.03 the most, because I like to make one thin line, and thicken if needed. I don't use thicker pens often because there is no way to thin the line afterwards.
I haven't found ANY cons really, so I'm here to tell you about the pros... Well, they are a little big, so if you like small pens/have a small hand, I guess you could count that as a con. The technical drawing pens are a bit large, but I like the feel in my hand. Their size enables them to have a bigger ink cartridge than the Prismacolor manga pens, which means they hold more ink, and will not dry out as fast.
This is a piece I inked with the Le Pen Technical Drawing Pen.
The last thing I've found about these pens is that I do not have to wait for it too dry before erasing the pencil lines underneath; it's ink soaks into the paper instantly. Less drying time, more drawing time.
I am convinced I will be continuing to use this brand and am very pleased with it.
So, to recap:
1) larger Ink cartridges = longer life
2) Even flow, no surprises
3) Many different sizes, great for details and larger projects
4) Drys instantly