I got the Full Circle to replace a cheap "under bridge" piezo that I wasn't satisfied with. I also wanted a more "upfront" sound with more focus on the strings than piezos under the bridge can provide.
I'm very happy with the sound of the Full Circle. It is a lot more focused, has better clarity and also bigger, tighther lows than my old ones. A lot less muddy.
The sound has a slight focus on strings rather than body. The G string is bordering on harsh but I wanted an upfront sound that would be heard in a (prog rock) mix so it's perfect for me. This may not be everyones taste though. There are certainly smoother sounding pickups out there! I should add that my bridge has threads up. Bridges with threads down are supposed to give slightly more focus towards the sound of the body with this pickup so probably smoother sounding.
Build quality... Well, it seems a bit fragile to be honest. I'm not confident that the wire going into the piezo will last forever. Also the design of the jack input never worked for me. It just kept falling off. I used an old sock + some duct tape to make another solution where I can insert it sideways under the tailpiece.
Installing it was very easy but obviously requires removing the bridge from the instrument, which is not something I would want to do on a regular basis.
I'm very happy with the sound, even though other pickups in the same price range can certainly provide a fuller/smoother sound (if that is what's needed). I would have expected slightly better build quality at this price but I guess it's not easy to make a robust design for a pickup like this so overall I'm reasonably happy with it.