I have a Fishman that I think is great for live performances. Certainly, the issue of whether you're abusing your guitar is an important one. In case you don't know, the transducer pickups that people are referrring to is this thin strip that goes under the saddle, requiring that a small hole be drilled under the saddle going into the guitar. the wire then connects to a 9 volt battery that is braced inside the guitar and goes out through the peg hole in the bottom of the guitar. The existing peg is removed, the hole is widened and the plug, which has the preamp built in, goes into the hole (you can also get them with separate preamps with or without controls). Do NOT try to do this yourself, no matter how handy you are. You need to go to an experienced Luthier (ie, guitar repair guy). It's a pretty easy job for the Luthier and it should not hurt your guitar at all. The hardware costs about $120 (or more if you get the fancier models) and the Luthier should cost about $100 or maybe even less.
I have the ribbon condensor and not the piezo type. As I understand it, the piezo type has these little chrystals that are interspersed to be under where the strings are, so there's more chance for a screw up if it's not positioned just right. One other thing - this will boost your saddle up about 1/8 inch, so if you're worried about your action being a tiny bit higher, you'll need to consider having the Luthier sand down the saddle a little.
[Edited by LI Slim on 09-19-2000 at 11:43]