Are feedback busters only for acoustic-electric guitars with the built-in mic? Or can it work even for the ones with piezo pickups?
I'm not too familiar with Ovation pickups eh.
I've experienced feedback before when I played my guitar (see my avatar) through an amp. I cranked up the amp, and when I got close, I got this distorted-feedback like sound. So having that feedback buster will eliminate this? 
most likely yes. i bought one from fastfret. cranking your acoustic guitar at louder volume would reduce the occurence of feedback.
Ovation, I think, uses a piezo pickup. Do i really need this nga even if Im using a piezo pickup ?
yes. my ibby acoustic has piezo PUs in it. i tried experimenting the feedback buster with the guitar plugged directly on the mixing console trying it at different volume levels. merong effect yung feedback buster, again when you want a louder volume on your acoustic.
the phenomenom of acoustic guitar feedback (positive feedback in theory) is similar to a microphone feedback. if the sound projected by speaker/s (and of course your acoustic guitar's sound comes out from it) gets back and received by your acoustic guitar's body through the sound hole (because of resonance), it is being picked up by the piezo PU, then amplified again, until the same process is repeated and repeated althrough out - which results in feedback. it's just the same putting your mic infront of it's output speaker. putting a feedback buster would somehow reduce its effect because you're blocking the sound hole by which the sound energy from the speakers goes in - cutting the effect of feedback.
same with stratman1, my comments on the feedback buster, medyo hassle lang ilagay at tanggalin. kaya ako dahan-dahan lang. medyo magaspang kasi... nagbabakasakali lang ba, baka kasi magasgasan yung finish dun sa soundhole.
but then i still agree with george lynch's quote courtesy of sir zasam. the player is still in control of his overall dynamics - and part of it is controlling feedback.
