I think your analogy is pretty good. I usually have a few loose g2s with me in case I run out of fountain pen ink in class. I find that gel pens sometimes make globs of ink, and that the lines aren't as nice as my FPs, but the writing experience is similar.
I do like the solid weight and feel of my fountain pens though, which is often lacking in rollerballs. The weight of the pen is the only thing that holds it against the paper the way I write, so light pens can be annoying.
Ur right about the weight. And I believe that is what a lot of people complain about in a rollerball. Which is why I use my Parker Sonnet. A good heavier rollerball is the cool looking Retro 51 (which uses Parker refills).
I do like the solid weight and feel of my fountain pens though, which is often lacking in rollerballs. The weight of the pen is the only thing that holds it against the paper the way I write, so light pens can be annoying.