Instead of buying that piece of crap, use the $ to get yourself to a dyno and see how much you can safely squeeze out of it. Then you can go to the track on a free night and see what she does. THAT is the way it should be done.
To answer your question, DON'T BUY IT! Those things are half-assed at best and you don't know how accurate they are. Of course, if you want to believe your moderately modded car is doing 12.5 in the 1/4 mile, then buy one to show everyone your results.