I have been using a clip-on mic on my fiddle - lovely sound but had feedback problems. I was worried about going for a piezo-based pick up because I have heard so many awful sounding fiddles.
However, good reviews and a good price led to me getting the Nanoflex and I have been very pleased at both the natural sound, and the feedback rejection. Fitting was straightforward, it does raise the action very slightly (but that will be sorted when I get a new bridge fitted), and it does very slightly dampen the unamplified sound of the instrument, but not noticeably.
Overall, I am happy to recommend it.