I have 3 of these installed on my Gypsy Jazz guitars. I needed something simple that required no guitar mod and would help me amplify my guitars without too much hassle/processing.
These I use only for amplification, the sound is NOT quacky (very important), the pickups take the sound from the body, close to the bridge at both edges.
Depending on your guitar you need to install the piezos in different locations to get the best from the instrument.
These are passive and send very little signal. You'll need a Pre amp. I've got a Harley Benton Pre amp and I'm golden.