Capacitors are fairly good at keeping charge - there isn't much loss over time. In fact, capacitors used in high voltage equipment should generally have a discharge resistor wired across them to deliberately discharge them slowly, to prevent them being a hazard to anyone repairing the equipment.

Superconductors don't help with this (much - just maybe with the wires leading up to a capacitor). Superconductors allow much better inductors instead. You can also store energy in an inductor, but it's different because in a capacitor the charge stays put and in an inductor the current is constantly flowing.