Great product. This is the third I have bought. Perfect for electro acoustic guitars when DI'd through PA, particularly when singing with mics where you need to hear yourself over the band and noisey crowds. Eliminates feedback, which allows higher levels in monitors. Essential when playing with drums and other electric guitars basses etc. Also handy for practicing unplugged at home at reduced acoustic volume.
Fits most standard acoustic sound holes but worth checking dimensions before you buy
I have seen so many acoustic guitarists struggling with feedback and stage sound over the years. This is a cheap simple solution that works. No brainer.