i had one awhile ago....in my old 03 gt.....it was damn accurate, i found a back road, and did a couple of 1/4 mile runs and got my time, it said a 14.23 and at the track my best for the day was a 14.10.....
i never got the horsepower setting right because i didn't know how heavy the car was....but it was definately cool, and measured your rpms almost exactly....
If you want an accurate HP for your car you can weigh your car at any big truckstop. Most truckstops have a platform scale that will give you weights accurate to within 20 lbs for each axle.