I love sketching with pens. Specially on a grey or beige paper, hatching it and then shading it with grey marker and white gel pen. For this, I used pigment liners for a long time but I kept having to buy new ones because the nib used to wear out before the ink did and even when it didn't, I had to use it in a particular position. I found it annoying.
And then I found a fountain pen lying around at my house. Of course it was mine but I hadn't used it for ages. So I googled how to get it to work again. And then after it did I started sketching with it. It was so smooth that I started using it for outdoor sketching. I started collecting fountain pens of different types and now I have a collection of different nib sizes including calligraphy pens and black ink, instead of the blue one I had been using.
It's absolutely awesome because unlike pigment liners I don't need to hold them in odd positions. And I can just add new ink in it instead of throwing it away and buying new. The pens might not be cheap but overall if you don't have to buy several new pens each month it's great. They can really last a long time and so does the ink bottle. There's not much choice when it comes to nib sizes but I have collected enough pens to solve that problem.
The first one I bought is a montex pen, I had a disposable one (which I used to experiment with calligraphy ink), I currently have one Calligraphy pen with an extra nib, a parker pen and Vettorio Fratini. The Vettorio Fratini is the only fountain pen I have with a thin nib and it makes fine lines which is what I was hoping for when I ordered it.
The montex is my favourite, it's also the cheapest one of the lot. It doesn't look like much on the outside in comparison to the other beautiful/sophisticated looking pens I have. It's sea blue and used to have cartoons on it which have rubbed off almost completely because it's so old, but I like it because it writes really smoothly. It's the pen that made me want to draw with fountain pens in the first place.
I also have a few dip pens which I only use at home since it's not feasible to carry them with me to outdoor locations.