One of the favourite pens of the Urban Sketchers Singapore group is the Hero pen (Hero 578, to be exact) with a curved nib. Hero is a Chinese brand (not Japanese) of fountain pens, and some of its ranges have curved nibs. I own two such pens - the 578 (top) and the 1077 (bottom) as shown below.
The great thing about this pen is the curve nib allows for different line widths, unlike normal fountain pens, felt-tip pens or ballpoints. The ability to produce line variation is a very desirable trait because you can now vary your line thickness without having to switch to a different pen and disrupt your workflow.
Line variation can be achieved through different means. For curved-nib pens, it depends on the angle with which you hold the pen. Flex-nib fountain pens and dip pens allow you to vary line width depending on how much pressure you exert, which varies the degree to which the nib tip splits (or opens up). Ballpoint pens also depend on the pressure you exert - more pressure pushes the point deeper into the paper resulting in a thicker line while less pressure results in a thinner lighter line as the point only glides across the surface. The Parallel Pen by Pilot and other flat-tipped calligraphy nibs depend on the angle at which the nib is in contact with the paper (different from the Hero pen as these pens' nibs contact the paper in a horizontal fashion while the Hero pens' do so in a vertical fashion).
Cathy Johnson, a watercolourist who has published several great books, reviews her Hero curved-nib in the video below.
It is to be noted that as with all fountain pens, it is not advisable to load the Hero pen with India ink as it will clog the pen and be very difficult to clean.
The Hero pens are very inexpensive. I bought my 578 in Singapore for just about $25 SGD and the 1077 was even cheaper when I got it in Malaysia for RM 11, which is less than $5 SGD.