Piezoelectrics are also used in those portable propane torches, of the type that Burnz-O-matic makes.
They have a finger operated trigger connected to an impeller so that when you press it down the impeller will strike a piezoelectric crystal, the voltage produces a spark in the nozzle of the torch which lights the propane. All the energy to light the torch comes from your finger pressing down on the trigger. With no batteries, the lighting mechanism will last virtually forever.
Heyup. They work great for that. Its the propane that actually gives useful energy. The crystal just provides a convenient spark.
Take away the propane and you just have a parlor trick.