IMO the biggest advantage isn't really anything objectively measurable. Yes, You can come up with things like smoothness, lack of resistance (my personal first

IMO the biggest advantage isn't really anything objectively measurable. Yes, You can come up with things like smoothness, lack of resistance (my personal first reason to switch), sustainability, but ultimately IMO You end up with "it just feels nice". To me, the main motivation to use them these days is that it simply feels good to use them. After spending some time tinkering / cleaning / nib polishing, fountain pens give me the same sort of pleasure that one gets from any other good tool - a well sharpened knife that just fits into Your hand, a good screw driver, a nicely worn in shoe, a well maintained / fitted bike... The list goes on. It becomes a sort of a self-sustaining feedback loop. The more You use one, the more You "want" to use one.