I have a g-tech pro comp, and I believe its fairly consistant with itself. I don't think you can trust it down to the last hundreth, but its consistant within a tenth or so of itself. Is it accurate, who knows. I know that based on what I've found, mine reads about 4 tenths slower than what the car actually does at the track. It said my STE did 15.33'* on the street, while at the track it did 14.96 with the same setup. (this was back when it was stock) I just use mine to figure out if something I did to the car helped it or not, howeverI know that if I run a 1/4 mile time I can expect the car to do the same time at the track. The key is the configuration, if you don't do that properly it'll never read right. While playing around with my STE last year, I figured out how to drop my 0-60 from 5.8 to 5.3 by just learning how to drive it a little better, so thats one thing it can help with, so you can figure out the best way to drive the car.
1990 Maroon Grand Prix STE McLaren 3.1 Intercooled Turbo, 14.695 @ 94.49 on 12psi before cam/heads/etc.
1990 Bright Red Grand Prix McLaren 3.1 Intercooled Turbo, 14.819 @ 92.72, 189whp
1999 Grand Prix GT 9.75:1|K&N|160 tstat|No Ubend/Cat/Res|UD Pulleys|Ported M90/TB/Heads|XPZ|2.55"|Pacesetters|Stage 2 I/C. 12.442 @ 112.78
2000 Blue Bonneville SSEi Supercharged Gutted Airbox|K&N|3.4"|1.9'*|WP Pulley|Custom DHP|180*stat|41932'*|No Cat. 14.681 @ 94.80
2002 Dark Cherry Grand Prix GTP Supercharged & Intercooled
12.084 @ 114.38 w/1.743 60', 315.6whp, Fastest Stock Cam M90 GTP!
NOW XP CAMMED, 340.6whp with XP, stock heads, Gen 3 M90, 11.984 @ 115.84 w/1.750 60'!
Most HP Stock Heads Gen 3 M90 GTP!
2004 Red Monte SS Supercharged K&N|3.4 Pulley|1.9'*|180*stat|41919 plugs|3" dp|No cat/ubend|DHP|Shift Kit. 14.2 @ 98
GOT BOOST??? Friends dont let friends drive stock!