I've had a G-Tech, and G-Tech SS.
With the SS it flashes a series of LEDs at different times during a run. ie 333, 660, and 1,320. Neither require a measured out distance.
All I use em for is comparison purposes, to see if the tune is better or worse, compared to the last run. While the SS has been blessed by carious mags as being pretty good, the 0-60' times are off, but overall the 1/4 mile times are about right. Trap MPH is always a little off, since it does an instanteous calc, instead of a timed run thur the lights at the end of the quarter.