Thusfar it's more of a novelty than a practical solution to our energy needs. The biggest longterm problem, IMHO, is the lack of mechanical robustness. If you want a device that, by definition, must repeatedly deform to generate a useful voltage, then that device must be extremely resistant to fatigue failure. Ceramics don't offer that, nor do the intricate interconnects that must be maintained in the contacting circuitry. When you start talking about putting it on roadways and other environments with frequent catastrophic damage caused by everyday use, the maintenance cost for replacing failed piezos would be huge.