For me it breaks down like this:
Pros:
- Fountain pens on average look nicer than other writing instruments. Especially the nibs, made of/with gold or other precious metals, are far more beautiful than the tip of a replaceable ballpoint or rollerball cartridge.
- Fountain pens are easier to write with, at least for me. I have to take a lot of notes during university lectures. For one semester I switched to a ballpoint, then to a rollerball, but I found both to be too much of a strain on my hand compared to a fountain pen.
- You can mix your own ink shade. (Ok, minor point, but I like it.

)
Cons:
- Using ink can be messy, both when refilling (depends on the mechanism) and when writing due to smearing. And you have to refill more often than you have to replace a cartridge.
- Most fountain pens are rather fragile, and not particularly "tactical".
- The nicer fountain pens are, in my opinion, extremely overpriced, considering they are usually made of very little metal and a lot of glorified plastic. I got an Omas Paragon very cheap here on German Ebay, because "Omas" also happens to mean "Granny´s" in German, and this makes these fountain pens hard to find among hundreds of "granny´s cupboard" and "granny´s stockings". I would never have paid retail for such a thing, you could almost get a custom folder for that price.
Kristofer