I've used it a few times and its accurate enough for the casual user.
Actual dyno tests on my car showed power to be above what was shown by dynolicious. (120whp dyno vs 100whp dynolicious)

If you want something thats more accurate you need a tool thats GPS based. The iPhone/iPodT accelerometer is heavily influenced by suspension flex, so a car with bad shocks will show as more powerful than one with new shocks.