I've got one of Ken's carbon fibre violin tailpieces, and I'm quite pleased with it. To underscore a comment above, these tailpieces are not a silver bullet. They do not make mediocre fiddles sound anything other than mediocre, just more definitively. Installing it on a quality luthier instrument has nice results: the same instrument is more responsive, has a qualitatively "cleaner" tone envelope, and a bit more punch. As a Baroque player, the carbon feels far more sturdy under my chin than does wood or Wittners. Most impressive is that the tailpiece makes it an order of magnitude easier to properly voice chords.
All that said, is it worth $350 for the violin model? Well, I spend about $125 on a set of strings, because it helps my instrument be more decisively itself. That amount of money is difficult for me to scrape up, but because this is my livelihood, I have no choice but to spend the money. For the cost of not quite three sets of strings, I have a non-consumable tool which helps me leverage my current instrument. So yes, I think the carbon is worth the cost, if one is making music as a livelihood.
Given that the viola, 'cello, and bass models are each much more expensive than the violin model, obviously a grain of salt has to be taken in translating my violin-specific comments to the larger instruments. I have no experience there, so I'd be interested in hearing your comments.
As a note, I'm not paid by Ken.