I was waiting for my son to finish with some activity last evening and had my portable kit with me, so I pulled out a bunch of nibs I had on hand that might qualify for semi-to-flex depending on your definition.
It was interesting writing with each one in immediate proximity to the others. It really pointed out some subtle differences in action from one to the other. I use the term "action" in the sense that it was used last century to talk about nibs: a kind of combination of flex and spring and stiffness. You can have a nib that springs back beautifully but doesn't flex very far. You can also have a nib that spreads far and wide but is still soft, like the Palmer 8. If you just judge by the width of line it doesn't really tell you much about what it's like to write with the nib.
A couple of things are obvious right off the bat:
1. My writing is NOT calligraphy.
2. It is definitely harder to have control with some nibs than with others
3. The Palmer nibs tend toward soft action and the Turner & Harrison are definitely on the stiffer side.
4. Nib size generally does not determine either flexibility, or action.
5. Some nibs trade spring and hairlines for smoothness, like the Eagle and Esterbrooks
6. The Spencerian 1 is really nothing that special. I found the Zanerian 3 and the Wm Mitchell "G" would make a much better pen for Spencerian writing, in my very ill-informed opinion.
These were all written on 25% cotton paper, 9mm spacing between lines, and written with a straight holder. All are vintage pens, some, like the Gillott 404 and the Turner & Harrison over 100 years old.
And, in the end, each one delivers a slightly different experience. The combination of nib shape, size, line, action, flex and smoothness add up to an overall experience. I enjoy them all, but some are most definitely more fun than others.