I have one. It measures horsepower (you have to enter total vehicle weight including passenger), lateral g's, 0 to 60 MPH time, 60 to 0 MPH time, 1/4 Mile ET and MPH.
It is accurate with some limitations. Due to it's inherent design (uses a silicon-based accelerometer) it can give different readings if you aren't on a level road, don't have the unit leveled out, etc. When all the adjustments are correct, you'll find it pretty much on the money.
It's fun to play with on the street.