I've tried using a handheld GPS that gave real time speed, but didn't record top speed (ie; you had to look at it on the fly). It seemed to give consistent readouts. As I recall, consistent reads were; carving a green run : 27 mph (easy to watch the GPS), carving a blue : 37 mph (harder to take eyes off run to see GPS), carving a black : 47 mph (could barely spare a glance at that speed), faster parts of the black : ?? (couldn't spare a second to look).
It matches dead on with a car's speedometer, too. Seems accurate on a plane, too. Pretty cool to see exactly where you are flying over...at 40,000 feet and 540 mph.