The instant you step on the gas they are wrong, as they have no way to correct for suspension squat. It is also very hard to get perfectly zero'd in. Power numbers will be useless as you can't compare them to BHP or dyno numbers. Still, at the price you got it for you really can't go wrong, but I think you will find yourself wanting more.
That "more" for me was found in a blitz powermeter-id.