i have used the mini flex for many years
and have several instruments outfitted with them./
the only draw back is that they are actual microphones
not pick ups or piezo/transducer types it can produce
feedback if you are in the position for the sound to
create a loop betwwen you, monitors, speakers, etc.

It has been great using this type of device, and
allows freedom of movement as well as ability to
fine tune the sound. i believe the end pin has to be
drilled a tiny amount to allow for the 1/4" plug to be
inserted. the new models that they are putting out
have user-controlled volume control right at your
finger tips. it requires a AA battery. These used to be
made locally and were invented by a friend of mine
who sold the business to GHS strings. they may have a
different name now...