I use a TWSBI ECO F (~$30) every day. It is one of the more inexpensive piston filled demonstrators out there. The writing experience is the best I've had so far (when paired with heavier Rhodia or Clairefontaine paper). The only downsides are (1) it uses a screw cap mechanism (which is preferred by some in the fountain pen community because it doesn't fail like snap-on cap mechanisms; but is slower and less convenient) (2) It's made of plastic -- very high quality plastic -- but plastic nonetheless. It's more of a technological pen than a classic pen. That said, I would rate the writing experience of the TWSBI much higher than that of the more popular Pilot Metropolitan (I should know, I own 4 Pilot Metros).

Also, as much as I'm a fan of fountain pens, they do have some downsides. The ink tends to smear/smudge easily (even quick drying ones) on normal paper, which makes them less suitable for scribbling marginalia on paperbacks. And even their extra fine nibs aren't that fine, so one can't squeeze as many words into the margins. Therefore, I use a different pen for marginalia: a Pilot Hi-Tec-C 0.3mm (~$3). It's a gel pen with a very very fine nib (0.3mm is finer than Western F fountain nibs). Fountain pen inks are also usually water soluble so they're less suitable for use cases that require permanence, like say signing the back of a credit card, so I find myself having to switch to a cheap ballpoint in such cases.

To me, fountain pens optimize for one experience and one experience only -- the pleasure of writing on good paper. It trades off versatility to achieve this optimization.