if you set it up correctly and use it correctly it's a consistant tool to measure how a mod affects your car
biggest problem is setting it up (original model was a PITA to mount and zero each run) and using it correctly involves multiple runs in both directions on a flat section of a road. drop the highest and lowest and average the rest
I use the Pro comp version and I like it. Works well with tuning. With the one you got just make sure you are on a very level surface and it is setup right and you will have a useful tuning tool
I have the Gtech Pro as well as the new Gtech Comp. I plugged them both in at the same time at the track. The older Gtech read an average of -.2 and 3mph fast, while the Gtech Comp was very very close to actual times. Had a friend buy the older Gtech and it read way off -.8 and 7mph higher. He sent it back and exchanged it for the Gtech Comp with no problems.
i used a buddy's on my car to test 1/4 times, and i didnt trust it. last time i was at the track i hooked it up again, and it was right on the money. the time slip said 14.695 @96.82 and the gtech said 14.68 @ 96.0. thats pretty damn close. i want to get one, but i rather put the $200 into making the car faster, not see how slow it is.lol
On the other hand, being able to test certain mods as to wether they help or not, as well as figuring out optimum shift points in order to shave off a few tenths in the 1/4 mile is $200 well spent. Doesn't take too many trips to the track,to eat up $200 bucks