If you're not a writer or artist, you might not find much practical advantage of a fountain pen over a gel pen; I might even advise against them. However if you do frequently write and/or draw, you'll find the customization to be unmatched: you can experiment until you find the exact writing feel you prefer, adjusted by ink, nibs, paper types, etc.
Since I found my match in fountain pens, I've found it impossible to go back to anything else. The small amount of maintenance (refilling and occasionally cleaning) is thoroughly outweighed by the tactile sensation of a writing/drawing instrument perfectly tuned to my preference. But I'm an amateur artist, so finding a well-matched tool is very important to me.
Since I found my match in fountain pens, I've found it impossible to go back to anything else. The small amount of maintenance (refilling and occasionally cleaning) is thoroughly outweighed by the tactile sensation of a writing/drawing instrument perfectly tuned to my preference. But I'm an amateur artist, so finding a well-matched tool is very important to me.