Got the first set of boards from Matt Venn for prototyping piezo preamps, and they're great! I've soldered both boards, after using the JFET matching described here (NOTE: link seems to be dead - I'll copy the circuit diagram from my notebook).
I'm constructing contact mics by gluing a magnet to the white side of a piezo disc, then soldering the preamp on, and gluing it (with some suitable insulation!) to the back of the magnet. Magnets are good for connecting with many metal structures, which often have good noises in. I think having a bit of weight behind the disc also helps to pick up more bass, but I haven't tested rigorously.
Comparative recordings to come.