I have not used a fire piston myself, but the guys who taught the matchless fire starting class at University of Scouting last fall tried to use one, with no success (i should probably mention they also tried with a bow drill set with no success either). Seems to me they are rather fidgety devices, with ongoing issues related to the gaskets required and as others have mentioned the required tinder as well.

From a gadget perspective, I suppose they could be fun to play around with, or good for demonstration purposes, but I wouldn't add it to my fire starting kit with all the other, more reliable methods around. Matches, lighters, fire steel, or flint and steel, seems like enough to me but maybe i'm missing something.