I used to use a pickup that replaced the bridge and had a small box with a 1/8" jack at the bottom (beneath the treble strings.) I was playing a Kohno in those days and didn't want to drill any holes; it worked well, but in my experience you have to turn the treble pretty much "off" to get rid of the quack. Later I put a Dean Markley pickup (semi-permanent putty stick-on type) inside a cheap La Patrie and found that by experimenting with the placement on the top I could get a much better sound than just placing it by the bridge, then I installed the pickup inside the guitar, in the same position. That way I could get a decent sound without getting too aggressive with the EQ and I must say, having an end-pin jack was really nice: no unsightly cables on the top of the guitar, just plug & play.
By and large I hate piezo pickups (it drives me berserk to see top guys like Paul Simon and James Taylor quacking away just to make things easier, when they got stellar sounds decades ago using mics) but you have to play the cards you're dealt; if you're playing a house party or an art exhibit, the sound of 100 people talking is very, very loud and any subtleties you might gain using a condenser mic often aren't worth the trouble.
Quick anecdote, once I played a Valentine's Day restaurant gig that was four hours long, using a Shure SM57 that had to be just inches from the guitar; at one point I changed position to relieve my anguished back and shoulders, and -BAM!- nailed the mic with the top of my guitar.
Further anecdote, the restaurant owner was an utter jerk who never talked to me, thus no food or discussion of breaks; eventually I just left and wandered to a gas station for a coffee, utterly burned out, and when I went to pay the cashier gave me an odd look . . . it turned out I had no money in my tux and had tried to pay with a worn-out piece of 1000-grit sandpaper. (He was nice and just gave me the coffee, plus a donut, double-win!) When I got back to the gig they were playing a Julian Bream CD, I wanted to kill them. -sigh-
2014 Michael Thames 650mm Cedar & IRW
1976 No. 10 Sakurai 660mm Spruce & IRW