This is my 25 cent pickup lecture. This generally means that you plug your pickup into an equalizer with a small preamp. IMHO condenser mikes are the best solution for this situation, since they have very little self-noise and a linear response curve from 15 to 20,000 Hz.
Condenser microphones are hideously expensive.
So piezo's are fine for amplifying your instrument on stage or connecting a tuner in a noisy environment, but don't expect a piezo pickup to produce a natural sound without additional shaping. My 0.02 Shillings.