No, but it looks pretty cool, and the reviews are good. The only gotchas I see are A) you need to know the exact weight of your car, and B) it's only accurate on level surfaces. If only 968s had OBD2 ports!
The auto-related iPhone app I use the most is V-Cockpit GPS, which is mildly useful, but absolute eye candy.
I also have G-Force, which seems pretty accurate as an accelerometer, but requires a particular installation.