They work really well for perfecting your launches and shift points, as well as for finding which mods work and what doesn't. The Hp figures will not be the same as on a dyno, as the G-tech will not be able to compensate for wind resistence, as not all cars have the same aerodynamics [in other words, the Hp figures will not be accurate, but they will be consistent] Also the Hp figures would be RWHP.
I take mine to the track all the time as I can then download my runs onto my computer when I get home, and if you set the roll out the same as what you stage at, then it is very accurate.