I fall into the same category as AC12. When I want to write with a flexible nib, I use a dip pen. There are thousands of variations of nibs available for cheap. (I've only paid $2 for a nib when it was a very rare vintage nib from the 1860's, most of mine I buy in larger groups so I get for cents rather than dollars)
I have a nicely flexible vintage Waterman and a modern Pilot with a soft XF nib. The Waterman is nicely flexible and comparable to vintage "college" dip nibs, while the modern Pilot is more like a vintage "firm" dip nib. But neither will get you the thin and thick variation of a dip nib. I've yet to encounter a flexible fountain pen nib that is as sharp as a dip nib. This is because fountain pen nibs are tipped, while dip pens are not. There may be a rare exception, but every one I've tried, and seen, can only get so thin in the hairlines.
What most people who want to get that beautiful modulation in line you get from a flexible pen don't understand before they start, is just how slowly you have to write to get that modulation. The more modulation (difference between the thick and thin portions of the line), the slower you write. Even Spencerian writing is mostly monoline except in specific strokes. Copperplate has more modulations per word than traditional Spencerian, but it's not a fast hand, and the modulation is not as extreme as in Spencerian.
If you want the modulation in line, but still want to write quickly and smoothly, then do what they used to to do way back when, get a stub nib. In the days of dip pens, stubs were viewed as useful for those who needed to write copious amounts quickly. The trick to making it look more like a flexible pen is to hold the nib so that the edge of the stub is parallel to the line of writing, rather than the traditional 45-degrees needed for italic writing. A fine or medium, smooth-cornered stub held this way makes for beautiful writing without having to take the time and expense for writing with a flexible nib.
On the left is me copying an old letter using a stub nib, and on the write is from the same letter using a flexible steel pen. (both are dip nibs) Kind of hard to tell the difference unless you look carefully.
Just a thought to throw into the lively conversation.
Edited by AAAndrew, 25 July 2017 - 13:11.