I bought the first model a few years back. I read a lot about it in mags. It's actually very... what's the word... consistant. The magazine articles I've read showed that the Gtech vs time slips times were slower. However, they were consistant. This thing will start timing when the car moves which is a few tenths before it actually breaks the beams.
My problem has been not finding a road that's flat, long enough or remote enough to use it on. The road has to be flat or you will not get an accurate reading. I've tried running flat out in some industrial sections of town, but you need at least 3/4 of a mile of straight, flat( no railroad tracks) road with no one around.
I hear the newer Gtech's are a little more accurate with a few more features but basicly the same.
There is also the problem of staring at that thing while you're flying down the street.