I have had one on my factory built J170 for about 5 months, I have flown around 35 hours on it and I could not be more pleased with it. The factory say about 3-5 kts speed increase, mine is nearer the upper end. I am told they resist rain very well, and the engine seems to run more smoothly.
The cost of the prop is about the same as a wooden one ($1500) but you have to buy a new spinner and backplate ($300)
The prop is made from black fibreglass, and looks great.