I really enjoyed the movement and rhythm of writing with a flex pen after getting the Noodler's Ahab.  Soon after, I was looking for more flexible fun so I decided to get the Pilot Falcon.  I know both these aren't true flex pens but I think they are good instroductions without getting into vintage.
 
Pen:       Pilot Falcon SF

Ink:        Pilot Iroshizuku Yama-Budo

Paper:   Rhodia No. 16 Grid

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