Jon, I have one, I just don't have the motor in the car yet!
I have heard one on a local WRX, though. It sounds fantastic! I think there is a sound file at www.rallyperformance.com
They aren't the prettiest exhaust out there, but they make really good horsepower numbers and sound better than anything else, IMO.
Most of the UK guys that have the exhaust love it and wouldn't change, IIRC.