Rob, I've never had one, but had many discussions about them with a guy who ran one on a Regal bowrider (on a 210hp 4.3L with a Volvo outdrive). For a long time (over a year), he absolutely swore by the thing, was happy with the performance, the cost, the quality of the product. Then, without explanation, on two different occassions, blades sheared off in the same way. He never had an impact (that he was aware of). The guy is a serious engineer and a bit of a product fanatic, so he did a complete 180 and started telling everyone who would listen that the product was defectively designed.
Personally, I think it's a great concept and I've heard other users say they are satisfied. Although I wouldn't have one as my primary prop, I wouldn't hesitate to use one as my spare. The flexibility of being able to change back and forth between an couple of prop pitches for $60 is also a nice benefit.