I have now tried these and when positioned correctly and with a little EQ can perform very well and are extremely feedback resistant. They are a tiny dymanic mic that adheres to the soundboard with a special putty and are designed to be temporarily installed each time you seek amplification. A few goes at the adhesion process and you get the idea. Used by Chris Thile from Nickel Creek amongst others. The drawback comes as the output is XLR female so you need an adaptor to use most guitar type pre amps which I would recommend. Schertler do their own version which goes with the DYN range of pickups. See price list.
Fantastic product from Baggs with very faithful mic -like reproduction. A microphone film is mounted on a small beam within the case. Two magnets dampen the response to give a robust, feedback resistant signal with warmth and natural dynamics. Gives a friendly, high output, don't have to EQ too much signal. This small transducer can be mounted permanently or temporarily on the outside of the mandolin or permanently on the inside (soundhole models).