I've brought Schaller Oyster S/P Piezo for recording the sound from a nylon string guitar as a hobby at home. Before I've brought it, I've read the reviews and watched some videos about this piezo, and everything seems to be acceptable.
However when I've received this little thing, I found that it is very hard to find a good place on my guitar to attach it to. It either sounds too loud, muddy, with tons of lower frequencies; or too quite, too sharp. At last I've found a balanced position for it, but I'm still not satisfied by the sound it provides. All the details will be recorded, such as some very tiny hand movement on the guitar. And if used with amplifier, the feedback is a serious problem. Since this is the first time I'm trying piezo, probably it is the common problem of piezos though.
After found the balanced position of the sound, I've tried to use double-side adhesive tape attached it to my guitar. And then I've found the cable really annoying, with such a long cable I could hardly put my guitar into the guitar case without squeezing the cable onto the guitar.
So at last I decided to remove the double-sided adhesive tape and used some adhesive pads (usually used for posters) to attach it to my guitar, which is cleaner than the "knetmasse" shipped with the piezo. Whenever I need to record it, I'll attach it to the guitar, otherwise I'll just remove it.
The finishing of the piezo is quite good, the metal shell seems to be very stable. However the problem is still the cable, it is very thin, makes one have the impression that it is going to be broken easily.
So to sum up. On the good side, the metal shell looks very stable, however the cable is a big problem. And not all guitars might have good sound recorded with it.